Hi, Wim!
In your formula this part "A2=A3&D2=D3" evaluates to a concatenation of 2 logical values, so it'd result in TRUETRUE, TRUEFALSE, FALSETRUE, FALSEFALSE values.
In Jason's formula that fixed part "AND(A2=A3,D2=D3)" evaluates to an AND condition between to logical values, so it'd result in "AND(TRUE,TRUE)", "AND(TRUE,FALSE)", "AND(FALSE,TRUE)", "AND(FALSE,FALSE)" values, that's to say TRUE, FALSE, FALSE, FALSE.
Hope it helps.
Regards!