It is normal for virtopia and other small pools. One day you hit 80 ravens the other day 200. This is a problem of smaller pools. When they don't hit a block, you'll not get paid. When a block is hit, you get paid after a block confirmation. When virtopia hits a difficult block, there is a longer period until you get paid, when it hits less difficult block, payments are more often.

I will try to calculate it for you, but those numbers are only rough estimates. Let's say the actual total hashrate on network is 1,200Th/s. Difficulty is adjusted so, that a block should be hit every minute. Meaning that all the pools get their share of blocks from the network according to their hashpower.  Let's say virtopia has hashrate 6.5 GH/s (last time i checked yesterday). 1200 / 6.5 ~ 185 So a block should be hit every 185 minutes on virtopia. When you get a block with difficulty 200% you have to multiply this number by 2 * 185 = 370 minutes and so on. If you hit a block with difficulty of 500% you won't get paid for almost 16 hours. However if you hit easy blocks with difficulty less then 100% you will get paid more then what you should be paid on average. Your hashrate is 20 MH/s, from the pools hashrate 6.5MH/s it is 1/325.  Virtopias pool fee is 1% So when a block is hit, 4950 ravens are distributed to the miners. With your hashrate of 20MH/s you should get 4950/325 = 15.23 Ravens per block, so only about 5 blocks have been hit during the last 24 hours that takes it to the 80 mentioned ravens per day. Those calculations might be a bit off, but you should be able to calculate the average from actual hashrate on network, pool and your rig.