Hi mocmclean,
Mattresses typically can vary +/- 1" compared to the ISPA industry standard dimensions for mattresses that are listed here.
There can also be some variation in the size of foundations or box springs between different manufacturers and it’s not uncommon at all that they are a little smaller than the mattress so that they can fit more easily into a bedframe (they can’t “squish” like a mattress).
Small differences between the width or length between a mattress and a foundation isn’t an issue at all and won’t compromise the support of the mattress.
Phoenix