Don't need 500 words to explain VARDIFF.  Stands for Variable Difficulty which means that the higher your hash rate, the more work the pool sends to your miner (worker).  Once you solve the work, all shares are sent at once then counted as a difficulty of one. It reduces the amount of traffic you need to send to the server.  It's unrelated to the current difficulty of the network, only related to the difficulty the pool sets to your miner.

Fluid - as in the Internet is a fluid network with sometimes it losing packets, capacity issues, network connectivity issues.  If your miner solves 1,000 difficulty 1 equivalent shares, but then goes to send them to the pool and there's a network issue, you could lose them.  Or if a new block is found or the server sends new work, the work you did on those shares is discarded.  All of that will be reflected in how the pool determines the hash rate.  Yes, therefore the hash rate is fluid. 

Keep your Satoshi.