However thorough the testing is, the bloke who's being tested in lab conditions is always going to be focussing very closely on not chucking. With every single ball he bowls he will be trying his very hardest not to chuck. That is his primary aim in the entire process. His career could depend on it.
That is not, however, going to be the case when bowling in a match. In a match he knows he won't get banned, or even no-balled, if he chucks it. So his focus quite rightly going to be on what he's doing with the ball, not what he's doing with his arm. He is intent on getting the batsman out. He may tend to strive for a little extra spin or (for a quickie) for a bit of extra pace. In doing so, or just through tiredness, he may unwittingly allow his action to slip a little bit.
Now I fully appreciate that the lab testing is rigorous, and they check (so far as they realistically can) that he's bowling in the lab as he does in a match, and do so over an extended period. But the bowler's mind-set in the two environments will be completely and utterly different.
I'm not accusing anyone of chucking, by the way. Just offering an answer to Daemon's question.