6 month thing flying from UK to Thailand

Recently arrived, but at Heathrow I was asked if I had been to Thailand in the past 6 months - I didn't question to find out - anyone know why this is - I thought the "3 runs" was the only restriction.

[globalwanderer]

Because the airlines are responsible for you if you arrive and are not admitted because you fail teh visa rules. I guess they are probably covered by asking and then only checking stamps if you say yes.. If you said no and subsequently found you had and were refused entry they'd pursue for their costs

Yes I know that, just wondered why 6 months though?

Yes I know that, just wondered why 6 months though?You can have 3 x 30 days in a 6 month period

I`ve been twice last 6 months via heathrow - never been asked that question. Both times with Thai airways - what airline did you use?