chain fire?

[Mr. 16 gauge]

O.K., I've been thinking about this for quite some time....hopefully someone can give me a logical & scientifically based answer.
I have read that you need to put a dolop of grease, lubricant, or whatever on a cap and ball revolver to prevent a ''chain fire", where a spark from a round will sneak in and set off all the remaining chambers at once.
Sounds logical, but riddle me this, Batman.......if I am using an oversized ball, and when I push that ball in the mouth of the chamber it shaves off some lead off the side, isn't the chamber "sealed" and safe from a chain fire? I mean, how on earth is a spark going to 'sneak in' and set off the powder in the chamber?
....and wont that dolop of grease potentially act as a bore obstruction, increasing chamber pressures?
I know we all do it, and I load & fire my Remington copy that way, but some of it just don't make no sense......
Enquiring minds would like to know...........