I think the reason is pretty much because you're only 20, and your beard is still developing naturally.

At 23, my 'stache compared to last year is looking better, but it still hasn't filled in completely yet, and still is very light. Also the middle of my 'stache has something yet to be desired, but I'm hanging in there.

I bet your 'stache will develop more. Some 'staches after the beard is fully mature, still are somewhat divided, but usually it's a very small division.

Just hang in there, Bro. The 'stache goat and sideburns look good.

Beards in olden times, were the emblems of wisdom and piety.
-Thomas Babington Macaulay

There's no wrong way to sport a beard, just like there's no wrong way to eat a Reece's.
-Danko