Conducted a lot of tests about this over long time. The conclusion come to based upon all or combination of the following, TA can be influenced by:
a) different terrains/clutter
b) antenna tilt (service area)
d) internal/external MS antenna
eg: 2400m from mast got TA fluctate TA22, TA24, TA25, TA28. Location in car park area with two-storey buildings surround MS. Antenna height on mast 21M. Mast located in semi rural area
eg: LOS 500m from mast on school buidling MS in open school playing fields TA1 and TA2
Maybe this helpful?