This is probably the Issue:
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
I know this is post is well over a year old, but in the very unlikely event you haven't corrected this issue or even check this forum, I figure I'd send a thought in your direction.
My guess is this is a much simpler issue than may appear. I would be willing to bet that there is a little bit of vertical space between each of your fields. If there is, that space will not disappear due to sliding -- only the vertical height of the empty fields will. So, with 2, 3, 4, or more empty fields, the vertical space between fields adds up to a noticeable amount.
A simple solution would be to increase the size of the fields so that the fields touch. You may want to set the text to center vertically within the larger fields.
If this gets to you... I hope it helps you -- or someone else!