Could be your housing. If you look at it from the side does it seem bent? also, it could be a swollen battery that is pushing your screen out like previously mentioned. If your battery is squishy or has wrinkles that might be the cause.
Also a lot of screens out there are Aftermarket. They are copy screens, so they might be a little thicker than the original. Do the screens from ifixit have apple logo under the cable? It it doesnt, its not original so the colors can look different and the screen might be thicker.
If youre picky about the screen fitting perfectly, i suggest you go to a shop that gurantees their screens are "oem" (with apple logo) or go to apple directly.
Oem screens for the 7 plus are around $70-$100 in the market right now for JUST the screen. If you payed less than 70 its an aftermarket for sure.