I would suspect a combination of factors, including when clocks change for BST/GMT. It may be that you fly out when there is a shorter time difference and back when it is a the more standard difference. I have the same in that I fly to JFK with a four hour difference but it is five on the return.

Return flights are also shorter due to the wind direction. This can easily save another hour or so.

There is also some possible rounding of times. I think between these factors this would explain the differences.

Edit : Mexico changes to daylight saving on 5/4/15.