I’m trying to understand if this is an error or is there something I’m missing?
According to the System reference document a megaraptor Fort save is +10. https://www.d20srd.org/srd/monsters/dinosaur.htm#megaraptor
As an 8 hit dice animal its base save is +6 because Fort is a Good save for it and it gains a +5 bonus on its Fort save due to its Con of 21, therefore its total Fort save bonus should be +11.
Edit: I also checked the D&D 3.5 Monster Manual errata and I notice the megaraptor is amended however nothing I can find resolves this particular issue.