A340-600's on LHR - BOS VS011

I have just logged into my booking and noticed VS have swapped the a/c operating my outbound flight to BOS next month from a A340-300 to a A340-600. The inbound a few days later is still showing as the usual A340-300.
I've noticed them putting -600's on this route a lot recently. Just wondered if there is any particular reason? Maybe higher demand or A340-300's being required elsewhere?
(The good thing, apart from now getting V:Port, is with more seats available they have moved my requested seats to some much better ones rather than being stuck in the middle.)
I've noticed them putting -600's on this route a lot recently. Just wondered if there is any particular reason? Maybe higher demand or A340-300's being required elsewhere?
(The good thing, apart from now getting V:Port, is with more seats available they have moved my requested seats to some much better ones rather than being stuck in the middle.)