The SPFL's the 18/20 split reasoning is a black box and seemingly justified subjectively and randomly. Reasons given in the past include:
Finishing in the wrong half of the table; least impact on the relegation fight; and now to rectify a previous imbalance.
We were disadvantaged in 2009/10, but benefitted in 2017/18 and so in overall balance. The reason given this time was to rectify our "advantage" in 2017/18.
It seems we were chosen this time because we would have least impact on the relegation fight (what about Hearts?), but also because it would result in the least number of fixture reversals. No guarantee then that we won't be fingered again.
What a load of codswallop from the SPFL.