Subtitle SubRip (*.srt) Positioning
A player doesn't have to acknowledge the position of subtitles. So a user could re-position them if he wanted to.
Thats actually the standard way of doing it. What you have there is a custom tag. The .srt format doesn't include any position data for the subtitles. Some players can recognize this sort of custom data and use it but you can't expect that a player will understand it.