Existing Project with Excel in WinXP

I'm just starting to recompile my applications on XP and I have VB6 SP5 installed. In a couple of projects that had Excel functionality, I noticed that the References had been automatically upgraded to Excel v10. But in one project when I look at the References, I noticed that Excel v10 was not available and that the checked Excel reference was actually v5.0. The full name was Microsoft Excel 5.0 Object Library and it was associated with XL5EN32.OLB.

Can anyone explain why this app didn't get Excel v10 or doesn't even have the object available? The other apps that got the automatic upgrade to the OLB file had v8 of Excel (at best). Is there something build in that does the auto upgrade for a two version upgrade but not for more? I didn't write this app and I haven't reseached it completely yet so I don't know all the Excel functionality, but I'm very concerned about using the v5 OLB file on an XP machine. But before I get the v10 OLB file into the project, I also want to make sure that VB didn't do the automatic upgrade for some other reason.

Jim