Interesting. When I looked up exactly the same journey last night on
my netbook via the TfL website it provided the fare and route via
Canada Water as the primary fare at £1.90 without any statement about
needed to validate at Canada Water. The £4.10 journey via Zone 1 was
shown as the alternative fare.

Today, using your link and looking it up separately, on a desktop PC
the £4.10 fare is shown as the primary route and the £1.90 fare via
Canada Water is shown as the alternative with a clear requirement to
touch on the route validator at CW. How very, very strange.

Is it possible you specified Canary Wharf on one occasion and Canary
Wharf [DLR] on the other? ISTR that differences of that sort can lead
to different primary fares/routes. And it does seem to for the route in
question this morning.
--
Robin
reply to address is (meant to be) valid