Anyone knows why there is big difference between current hash rate and average hash rate?

I used old rig with RX480 8G to mine at flypool, but the average hash rate is far below current hash rate. Don't know why? Does CPU also take part in the mining process?

Btw the date is below:

Old rig CPU E8400

Current hash rate: 270.0H/s
Average hash rate: 17.2H/s

Highend Rig CPU E3 1270 V5

Current hash rate: 247.5H/s
Average hash rate: 294.1H/s

The average rate is the average rate of the last 24 hours so if you didn't mine the full 24 hours you will have a lower average and the current rate will always be a bit higher lower at any time

Another factor (though not usually a big one) is choosing a pool server as close to you as possible to lower the ping time.

For example - I connect to the asia1 flypool server because I live in Australia, which gives me a ping time usually around 50-60ms whereas if I connect to the usa1 server it can be 200-400ms. If your ping time gets really high, I think you are much more likely to produce stale shares.

I think though as mentioned above - you can't really tell what hash-rate you are actually achieving until you have been mining constantly for 24 hours or more.