<rdar://problem/7759909> Due to the limited precision of the float type, with word- and run-rounding enabled, with certain fonts, measuring individual words in a text run and adding up the widths gives a different result from measuring the entire run.
Created attachment 61567 [details] Fix
Fixed in <http://trac.webkit.org/projects/webkit/changeset/63357>.
This patch claims to have been landed.