doesn't that seem odd to you?
this is a plausible scenario:
you boil your priming sugar, cool it, add it to the bucket, and rack the beer over into the bucket. you see it swirl, and think it is mixed up well enough. in the past it was good enough, so it should be good enough this time too. unfortunately, it is entirely possible that the priming sugar didn't mix well enough, and when this happens, some beers have the exact right carbonation, some are flat, some are overcarbonated (gushers), and others are bombs.
again, i would say to be careful, and use liquid proofing around the bottles just in case. this is one of those things that it is better to have it and not need it, rather than not have it and need it.