Positioning WebVTT closed caption lines. (In TextEdit)

So, apologies, this isn't exactly a Premiere thing but relates and could possibly be useful to remembers of this community.

I am testing the positioning information of WebVTT files used for closed captioning on YouTube. In the demonstrative example video provided below, I've captioned the video myself by editing a WebVTT file with "TextEdit" on a Mac.

Test video: https://youtu.be/h300uF_Djcc

All I did to my file was reposition the caption lines to display at 2 different heights in the screen. I did not make any changes to the first or last line, but I did change the position of all of the other lines to the same thing.  I set all of these lines to what should be the same position by adding "line:70%" after the timing information in the file. 

Now, if you look closely you'll notice some differentials.

Ignoring the first and last lines, you'll notice that the other lines seem to be positioning near eachother but in 2 slightly different locations. The lines that are shorter seem to display higher and the longer lines display lower even though all of my captions are single lines and contain the same positioning information. Does anyone know why this could be?