Mask is showing background frame border

[Jordan28783208l6ct]

I'm a novice at masking in Premiere, but have been messing around with a personal project to try and learn different effects.

I was able to create the effect I wanted using 2 masks on this image to get rid of the background, but when I moved the position and zoomed the masked image out, I noticed that the original frame lines are visible for some reason? Can anyone help me figure out why? Thanks!

[Jordan28783208l6ct]

You can't really see the frame border well in the screenshot, but I just used a 0.1 or 0.2% crop on the edges with the borders and that seems to work fine. I'm still unsure of why there was a border to begin with though.