I think kozlodoev beat me to it, but I'll still reply with my opinion :p
My shooting guards has these skills. How comes he ended up shooting SG M. Manganelli 53 15 - 39 6 - 18?
He played an awful lot of minutes and his stamina is only inept, so he would've been absolutely knackered come the end of the 3rd, let alone the fact that there was OT. By putting in a backup for him, the coach would give him time to rest every now and then, and you will likely see an improvement in his game stats.
The first 3 quarters he only played 9 minutes and his backup who is a lot weaker played all the time. Why can/would this happen when I set my coach orders to strictly use depth chart?
Your starter C got himself in foul trouble in the 1st Q so the coach took him off. Similarly, he fouled again in the 2nd and the coach took him off again so that he wouldn't foul himself out before the 4th Q, where you needed him to stay in the game. Either train his stamina, ID or wait until he gets more experienced and he will probably get into foul trouble a little less, or you can now use the option of letting these guys play even if they foul a bit too much - just change the 'foul trouble' option in the tactics from "sit them" to "let them play". Bear in mind that if this guy gets into foul trouble often, he may well get fouled out before the end of a lot of games unless you let the coach bench him for a while, and you may end up in serious trouble at the end of games when you really need him (the game in question is a good example).
Just as an extra piece of advice - looking at your team and his team, it's clear that you have a stronger roster and it may have been better to use a faster pace tactic to really drive home the HCA, although I can understand that with no backups for your guards this may have worked against you, even though I think you would have clinched it in regular time.