Dataset.reduce fails wheras Dataarray.reduce succeds

What happened?

I tried to reduce dataset using scipy.stats.skew, but it raised an error.

What did you expect to happen?

I expected it to succed, especially because the same operation with dataarray succeeds.

Minimal Complete Verifiable Example

>>> dataset = xr.Dataset(
...     {
...         "math_scores": (
...             ["student", "test"],
...             [[90, 85, 92], [78, 80, 85], [95, 92, 98]],
...         ),
...         "english_scores": (
...             ["student", "test"],
...             [[88, 90, 92], [75, 82, 79], [93, 96, 91]],
...         ),
...     },
...     coords={
...         "student": ["Alice", "Bob", "Charlie"],
...         "test": ["Test 1", "Test 2", "Test 3"],
...     },
... )

from scipy.stats import skew

# Calling DataArray.reduce on each variable individually succeeds

In [14]: dataset['math_scores'].reduce(skew, dim=["test", 'student'])
Out[14]: 
<xarray.DataArray 'math_scores' ()>
array(-0.19423043)

In [15]: dataset['english_scores'].reduce(skew, dim=["test", 'student'])
Out[15]: 
<xarray.DataArray 'english_scores' ()>
array(-0.60125)

# But calling Dataset.reduce fails
dataset.reduce(skew, dim=["test", 'student'])

--------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[16], line 1
----> 1 dataset.reduce(skew, dim=["test", 'student'])

File ~/Documents/Work/Code/xarray/xarray/core/dataset.py:5972, in Dataset.reduce(self, func, dim, keep_attrs, keepdims, numeric_only, **kwargs)
   5955         if (
   5956             # Some reduction functions (e.g. std, var) need to run on variables
   5957             # that don't have the reduce dims: PR5393
   (...)
   5965             # the former is often more efficient
   5966             # keep single-element dims as list, to support Hashables
   5967             reduce_maybe_single = (
   5968                 None
   5969                 if len(reduce_dims) == var.ndim and var.ndim != 1
   5970                 else reduce_dims
   5971             )
-> 5972             variables[name] = var.reduce(
   5973                 func,
   5974                 dim=reduce_maybe_single,
   5975                 keep_attrs=keep_attrs,
   5976                 keepdims=keepdims,
   5977                 **kwargs,
   5978             )
   5980 coord_names = {k for k in self.coords if k in variables}
   5981 indexes = {k: v for k, v in self._indexes.items() if k in variables}

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:2045, in Variable.reduce(self, func, dim, axis, keep_attrs, keepdims, **kwargs)
   2042     keep_attrs = _get_keep_attrs(default=False)
   2043 attrs = self._attrs if keep_attrs else None
-> 2045 return Variable(dims, data, attrs=attrs)

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:367, in Variable.__init__(self, dims, data, attrs, encoding, fastpath)
    347 """
    348 Parameters
    349 ----------
   (...)
    364     unrecognized encoding items.
    365 """
    366 self._data = as_compatible_data(data, fastpath=fastpath)
--> 367 self._dims = self._parse_dimensions(dims)
    368 self._attrs = None
    369 self._encoding = None

File ~/Documents/Work/Code/xarray/xarray/core/variable.py:683, in Variable._parse_dimensions(self, dims)
    681     dims = tuple(dims)
    682 if len(dims) != self.ndim:
--> 683     raise ValueError(
    684         f"dimensions {dims} must have the same length as the "
    685         f"number of data dimensions, ndim={self.ndim}"
    686     )
    687 return dims

ValueError: dimensions () must have the same length as the number of data dimensions, ndim=1

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

xarray main branch

Start with the reproducer using scipy.stats.skew, then inspect Dataset.reduce in xarray/core/dataset.py and Variable.reduce in xarray/core/variable.py. Compare the successful DataArray reduction with the failing Dataset reduction and verify that the dataset result preserves compatible dimensions for both variables.