Ore extraction bug

[Ksp Slingshooter]

I have an uncrewed miner with 6 drill juniors that is mining an asteroid at the supposed rate of 0.06 units of ore/s (which would mean that my ship would be gaining 0.6Kg/s while the asteroid loses that amount) but the asteroid is losing a bit more than 1Kg/s.

At this point I thought that the junior drills would be very inefficient but no, something even weirder is happening: the total mass is increasing. 

To sum up, the drills are extracting at lower than expected rate of ore and therefore my ship is gaining less than expected rate of ore, the asteroid is losing ore at a higher than expected rate, but the total mass of ship + asteroid is increasing (there's no conservation of mass)