I could be wrong but I think this toggles based on if you use the special field of "Page N of M" in your report.
If you use that field it will calculate the total number of pages and display that as "1 of x" . This also slows down your report because it has to calculate this field BEFORE it displays anything so it makes an extra pass on the data before display.
If you do not use it it will show the 1+.
Give it a shot to see if this guess is correct.