I would need to see two enlarged versions to be sure but that's not the only difference in those pictures. I think they removed a number of ships in that picture to make the water look cleaner and less cluttered. If you look at the full poster (Warning: Daily Bile link)
http://www.dailymail.co.uk/sport/olympics/article-2059345/London-2012-Olympics-poster-HMS-Belfast-removed-image.html and look over the entire river there are far fewer ships than would be expected at any one time on the Thames. Especially further down the river.
The mistake could well have been also removing HMS Belfast by a designer being oblivious to the reaction it would provoke.