Re investigate contiguity checker time coordinate length discrepancy

The error below previously seen in #931 has now occurred for `ua_tavg-al-hxy-u` and `va_tavg-al-hxy-u` in the ap6 stream. This should be re investigated to determine why the initial fix does not work for these variables.
```
{
"mip_table": "atmos@day",
"checker": "Time contiguity check",
"error_message": "Total length of time coordinate, 3637, is different from 3653 implied by time bounds (from 2000-01-01T00:00:00Z to 2010-01-01T00:00:00Z) and time frequency (P1D)",
"affected_files": 2,
"affected_vars": "ua_tavg-al-hxy-u|va_tavg-al-hxy-u"
},
```

Review the initial fix from issue #931 and the Time contiguity check for the ap6 stream. Reproduce the discrepancy with ua_tavg-al-hxy-u and va_tavg-al-hxy-u, using the reported coordinate and time-bound values as the starting case. Done means the underlying cause is identified and the checker no longer reports an incorrect length discrepancy for these variables.

Written by the indexing model from the issue text.