A question on understanding the notion of VESTS

Hi, I am new to steemit and trying to understand how it works.

I believe I have figured out the relationship among SBD, STEEM and SP. However, I feel hard to understand VESTS. It looks like when I decide to lock 1STEEM for 1SP, I automatically get about 2000 VESTS. The ratio between STEEM and VESTS can be found at steemd, which is 0.000495302 SPV right now.

My question is, what are the reasons that make the value of SPV keep fluctuating rather than being constant. If the VESTS assigned to every new deposit or withdraw of SP is computed with the current SPV, then it looks like there should be no fluctuation of SPV.

Sorry this may be a naive question but your help is greatly appreciated.