Why would LVM_GETCOUNTPERPAGE return incorrectly?

I'm using the LVM_GETCOUNTPERPAGE message to determine how many rows to add and fill the ListView, but it's returning a value that's too high every time. It's off by 3 rows and I can't figure out why. It's got LVS_REPORT and LVS_EX_GRIDLINES styles set. It almost seems like it's not accounting for the gridlines. Is that possible?

Code:

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int PageSize = SendMessage(hMemData,LVM_GETCOUNTPERPAGE,0,0);
    for (iRow = 0; iRow < PageSize; iRow++) {

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Need more info....

When are you sending the msg?
Do you have the right HWND?
Have you compared the size of each row (from LVM_GETSUBITEMRECT)? (ie if you divide the client area by the height of a single cell)

Post the minimum code that demonstrates the problem.