If it was placed after the cut off point for next day, then that would be why. The order would be placed on the Friday, processed and picked up on the monday.. I would imagine the delivery delays are due to the snow!
EDIT: Also, if you order on a friday and want
true next day delivery you have to pay for saturday delivery. I believe that the options for standard delivery is 1-2 days (unless you paid the extra for next day, which I thought was more than £7.95 when done online), which would put it right for a tuesday delivery as they are working days, rather than actual days.