tREFI result in simulation and on the board might be larger than expected if you set tREFI to less than 7.8us in DDR/DDR2/LPDDR2 MegaWizard.
DDR/DDR2/LPDDR2 SDRAM IP has MEM_TREFI parameter which defines tREFI parameter in terms of memory clock cycles.
Since the minimum value of this parameter is limited to 780, tREFI becomes larger when memory clock is slower.
For example, tREFI for DDR2 SDRAM should be 3.9us at >85C. But if DDR2 memory clock is 125MHz(8ns), minimum tREFI value can be 8ns x 780 = 6.24us.
tREFI for DDR should be 7.8us. But if DDR memory clock is 76.9MHz (13ns), minimum tREFI value can be 13ns x 780 = 10.14us.