Average based on a condition using OR operator giving wrong results

Question

I am trying to find average of a column based on the values from other column with different array formulas :

Using boolean +

=AVERAGE(IF(($F$2:$F$390="A")+($F$2:$F$390="B"),$D$2:$D$390))

Result is 40 and it is correct

Using Curly brackets inside IF

=AVERAGE(IF($F$2:$F$390 = {"A","B"},$D$2:$D$390))

Result is 40 and it is correct

Using OR Operator

=AVERAGE(IF(OR($F$2:$F$390 = "A",$F$2:$F$390 = "B"),$D$2:$D$390))

Result is 37 and it is wrong.

Could anyone help me to find, what mistake in the 3rd method using OR operator is giving wrong result ??

(PS : All are array formulas executed using Ctrl + Shift + Enter )