True in 99% of cases.
I witnessed at least two nondetermenistic expansions:
1. Expansion went to system 24,39ly from expansion source, skipping system 24,03ly with free slots. It was in April, prior to 2.1 however.
2. System 21,01ly away from source was picked instead of system 20,65ly away. Both systems had 6 factions before expansion. It was in December.
The simplest explanation is that BGS used rounded to integer distance to pick expansion target.
Prior to 2.1 the expansion rules were quite different.
IIRC:
1. Look for system within 25ly with less than 5 factions. Barring that,
2. Look for the faction with the lowest influence in that radius, and expand to that system and fight them for position. This sometimes led - in fact it was the only way, I think - to native factions being pushed out of their home system, and thus the confusion since about a few remaining faction that haven't been forced into Retreat, which is the usual way for them to return to their native system.