This is because TfL decided to make any travel originating or passing through Zone 1 stations to or from Heathrow peak at all times. This is probably primarily designed to hit holiday makers rather than those commuting from the western suburbs as many airport employees do.

If you play the Hatton Cross trick during off-peak periods, it's still only £3.60 going to or through Zone 1 stations. If you're only travelling as far as Zone 2 (e.g. Hammersmith) then you don't need the trick unless you want to save a few 10s of pence originating from Zone 5 instead of Zone 6.