Antenna gain calculated by Ansoft HFSS 8

when i compare the antenna (dielectric resonator antenna) gain calculated by Ansoft HFSS 8 with the measured results, it shows that the calculated results are 1dB higher than the measured ones in the boreside direction.

How do you think of the reason behind this phenomenon? What's your comment on the capability of HFSS in gain calculation?

thx

it can cause by some reasons :

1. your radiation box dimention be not correct .
2. your input feeding line doesn't be exactly match the same as practice .
3. your bounding assignment does not effitient .