Just look at this!

Mally had dealt. £250,000 against £100.
Offer: £145,000

Kirsty hadn't dealt. £250,000 against £75,000
Offer: £125,000

Well it's well known the banker offered alot more in the early days of the show.

Also remember in the early days of Deal Or No Deal:

Jennifer final two boxes

£750
£250,000

Banker's Offer: £120,000

But months later:

Morris final two boxes

£20,000
£250,000

Banker's Offer: £101,000

I don't think it's only to do with the timing. Surely the character of the player, including their perceived expectations come intot he banker's calculations. Jennifer and Mally were poles apart, so I'd expect variation in the offer value.