HELP!!! rear speaker question

Need help please, curious as to why my girlfriends 93 integra rear speakers are NOT 6.5 inch.....they are exactly 6.25, as is the speaker mount they rest in. Any true 6.5 inch speaker will not drop into place, but instead rest on top of the speaker mount. I would like to put in a complete new system for her, but would like a more air tight seal than simply resting the speaker on top. Am I the only one who has a ackward speaker size.
The fronts are also 6.25 inch. The odd thing is the 6.5 inch speaker do allign with the factory holes but will not drop into the base of the mount, they sit about .5-3/4 inch higher than the base itself.
Any suggestions
Thx

I had a 91 Integra and had this problem.

I got me a hammer and fixed my problem. They will cram in there, I didnt even screm mine down in. It did require the bending of the three little stupid tab things that sat on the edges but they are useless so it didnt matter.