Im guessing its because of a few things colour/spec but also allocation at dealers when they actually put there order in for example.
Just been told mine is provisional week 48

I was told this could change until 4-5 weeks before the build week which is the time when it gets confirmed.
So far we have (i think)
JULYHERBIE911 July 3rd BW 41
ROLFE July 7th BW 35 (arrived)
SPARTICUS July 7th BW 42
VW KEV July 21st BW 40
GAZDEBAZ July 22nd BW 42
DONNIEDARKO July 30th BW n/a
AUGUSTMDSS MK6 Aug 12th BW n/a
FREAKINSWEET Aug 12th BW 44
LIVERLEGEND Aug (mid?) BW 48
SNOOPY Aug 20th BW 48 (not confirmed)
STEVEN_9709 Aug 22nd BW 47
MB26 Aug 27th BW 47 (confirmed)
I was just thinking / wondering as mines not till week 48 and then 2 to 3 weeks later probably before i can collect it (if i can get time off!), i wonder if i could pay for it before the end of the year (for current tax rate) and get them to register it maybe 1st jan or get them to register it before end of the year but only tax it 1st jan? saves wasting a months tax.