Why does class of service availability change on a particular flight when putting in different connections. Example: BOS DCA PHX on July 25th: Flying on US 2142 and connecting to US 563, EF shows F9 and X5 on flight 563. Flying on US 2139 and connecting to US 563 (the same flight from DCA to PHX), EF shows F8 X1 on flight 563. Doesn't make sense to me! Any explanation? Thx for your insight!