Hi Kcx,
There are some comments about standard sizing and sizing variances in post #2 here that should be helpful.
It can take a few days or in a few cases even up to a week for a mattress to completely decompress and regain it’s correct size within a normal spec tolerance of +/- 1". While it won’t affect the comfort or support of the mattress … assuming that you’ve measured the mattress correctly and it’s still out of spec after that I would call the manufacturer or retailer you purchased from to let them know that your mattress is out of spec and they should be able to help you.
Phoenix