It's because the valor you receive from the match in which you level is already applied to the next level. But it's just the valor from that match. As you said you get out of a match and your valor bar is 1/2 full. If it was actually stacking you would get out of the match and it would be completely full, and when you dinged 50 you would see it jump up to like VL 54. It only takes like 3 matches to max valor level even at mid 40's, so it would make sense that as you leave the match you have 1/3 of your progress to the next level. Just as it also makes sense that if it only takes 3 matches to max valor, but 10 matches to go up a level you would have a LOT more saved up than just 1/3-1/2 of your next level if you in fact had 40 levels worth of stacking going on.
That would make sense. But is valor that easily obtainable that you can literally get half a bar from one match? I've had this happen... more than once. I assumed it had some kind of cap but beyond your level, or even a DR to where you're getting SOME of the valor past your level stacked but not all of it. It seems like a little much to have about a half bar into the next level.