Considering that this was apparently one person doing investigative research, and a very small fraction of each code base (freebsd apparently has about 9 million LOC, openbsd 2.9 million), I do not think it is reasonable to state that freeBSD "only had 20% more bugs", but rather only that the investigator found 20% more bugs in it in his research. He probably examined a similar number of LOC of each OS and spent a similar amount of time on each OS if I had to make a guess -- the slides do not say however.

If he had spent three times more time on freebsd than on openbsd I would agree with you.