Weird results from mapdist()

Hello,

I am recently using mapdist() to calculate the distances between some cities in Taiwan:

``` r
library(ggmap)
from <- c("Taipei", "Taipei", "Taipei")
to <- c("Taichung", "Tainan", "Kaohsiung")
mapdist(from, to)
```

The results make sense to me:
Taipei > Taichung 165 km
Taipei > Tainan 310 km
Taipei > Kaohsiung 355 km

However, when I did the reverse, I got the following results:

``` r
from <- c("Taichung", "Tainan", "Kaohsiung")
to <- c("Taipei", "Taipei", "Taipei")
mapdist(from, to)
```

Taichung > Taipei 311 km
Tainan > Taipei 164 km
Kaohsiung > Taipei 356 km

The distances themselves are still correct, but they didn't line up with the city pairs. I wonder why this could happen. Is there any way to solve the problem? Thanks!