I asked someone else who has pretty good insight on what goes on in the booth on Sunday the same question and he said this:
During games, tackles, like other stats, are kept by the statistics crew hired by each team. Coaches look at film and keep their own set of tackles and some teams, like the Texans, use tackles compiled by their coaches. None are regarded as official. In truth, the coaches are too liberal and the stats crews are too conservative.
In other words, take the Tackles stat with a grain of salt.