BTC amount yo-yoing?

I've got a few units on my account here, and 3 times today I've noticed an oddity. I have it set to earn on BTC presently, and it's gone just up to & over $80 earned, then when I refresh the page (few minutes later, half hour later, etc) it 'reset's down to ~$77 value, then re-earns back up to $80. Just watched it do it again.

Anyone else having this issue?

You are getting paid in bitcoins.
If you have .1 BTC when the bitcoin price is 5000, its worth $500.
If you have .1 BTC when the bitcoin price is 5300, its worth $530.

As the price of BTC goes up and down wildly like it has today the value of the earned coins you have will flux throughout the day.
That's why I always examine the TOTAL EARNINGS bar graph. It shows how much $ I earned for the hashing, not how wildly the coins I am choosing for the payout fluxed during the day.

I understand the fluctuations like that, but considering BTC is basically skyrocketing today, it wouldn't make much sense for the value per BTC earned to drop, does it?

If it was only going up then sure you would be correct.
At 12:30 it hit close to 5400, by 2:15 it was down near 5200 and then close to 3 PM its back to 5350.

So yes each time it goes up or down the price for the amount you have collected so far will change also.

What's the refresh rate? Every minute, five minutes, hour, etc? Does anyone know?

I would guess on the LIVE BALANCES, if you click on the coin and expand it, on the right hand side it says how long since the last update/process period. Sometimes it can get way behind depending on the system or errors, usually within 30 secs when its running smoothly I've seen.