it can not be due to variance??? Who's turning on new miners? New Chinese mining farm(s)? Don't they know btc price is in the dumps???
It may be possible that people mining other sha256 altcoin are changing their miners back to Bitcoin. It may also be the luck, the network happens to be mining more blocks at a faster rate due to the luck or perhaps mining companies testing their new ASICS. The network total mining hashrate is only for estimation only, it is not 100% accurate as no one can count the machine's hashrates accurately.