Random number generator, what's wrong with my approach/statistics? [JS]

First of all, what I want to know is if I am doing a systematic fault or am I messing up with the math, or do you have any ideas what could be wrong?

I was trying to write a little random number generator which numbers can be influenced / verified by the user (provably fair). The generated number is between 4096 and 65535. I simulated the function 100,000 times and get some strange statistics (Or am I thinking wrong??). Why is the chance that the number is under 8000 around ~50%. Shouldn't it be 50% around the middle of the number range (~30700)?

Here is the output of the simulation:

< 65536 : 100000 Times :100%
< 60000 : 91813 Times :91.813%
< 56000 : 86406 Times :86.406%
< 52000 : 81334 Times :81.334%
< 48000 : 76743 Times :76.743%
< 32768 : 62356 Times :62.356%
< 32000 : 61748 Times :61.748%
< 30719 : 60860 Times :60.86%
< 24000 : 56628 Times :56.628%
< 16000 : 52871 Times :52.871%
< 12000 : 51540 Times :51.54%
< 8000 : 50447 Times :50.447%
< 6000 : 36003 Times :36.003%
< 5096 : 21583 Times :21.583%
< 4608 : 11714 Times :11.714%
< 4250 : 3674 Times :3.674%
< 4100 : 100 Times :0.1%
< 4096 : 0 Times :0%

A little more details on the function I wrote:

I am generating two hashes. One is the userhash and the other is the serverhash. Both of these have a format like e.g.:

Server =3CDC3C8C97DEE62169B2C403BB2B6B501C1B0A0BD8015699B47DA67789C3F628
User   =CF27CC73E33E0AC1DA5239DE8DAF94044D89B8636DA90F4CE510E652C8AC7F54

(The Userhash, is generated by a unique ID. The Serverhash is generated by taking a random number (standard function: math.random() )and a timestamp and then HMAC-SHA-512 them.)

To get my "random" number, i take both (user and server) hashes and add them as hexadecimal numbers (userhash + serverhash = result). Then I take the result and cut everything except the first 4 digits. (E.g result = CF27) After that, I convert it to decimal again.

The smallest number I can get from this should be in Hex 1000 (dec=4096), the biggest in Hex FFFF (dec=65535). That means my random number should be in a Range of 4096 - 65535.