Meat probe placement can be pretty important in accurately tracking meat temps. Try to avoid large fat pockets, don't stress about the point, it will do fine. Make sure that your probe is near the center of the thickest part of the flat.
Internal temp is just a gauge. Once it reaches 190* start regular testing with a probe in the flat. (Toothpick, ice pick, thin thermometer probe). You are looking for very low resistance to probe insertion. (It is a learned feel).
Until you develop a reliable feel for the probe test a dependable dual probe thermometer is your very good friend.
Don't sweat smoke chamber temp swings too much unless they go above 300 degrees, and even then with "long term smokes" , (Like brisket), don't fret unless the temps stay elevated for over an hour. It will work out OK.
As WHB says, "the supplied thermometer with most smokers" is at the least suspect. A good budget minded therm is the dual probe Maverick.
Every hunk of meat is going to act as it's nature dictates and so it is very difficult to tell you exactly why your brisket turned out as it did.
Experience is going to be what it takes to develop a feel for each cut of meat but as long as your not having to throw away your practice smokes your doing good. (I am sure that there have been plenty of "first smoke" briskets that became dog food). It is one of the more difficult cuts to get a real grip on.