Topic: Retrieving keys using MFOC : distance varies significantly

I've been trying to use MFOC along with my ACR122U reader to retrieve the keys from my MIFARE Classic 4K card. When using MFOCi it indicates that the card uses some of the default keys, two of them twice. And then starts to try to crack the others using one of these (initially the default fffffffffff… key). Unfortunately even with the number of probes set to a very high number like 20000 it never succeeds.

Noticeably the distance returned for each probe varies significantly, going from up in the 40000s down to the high 10000s — I assume this is problematic given screenshots from others whose distance is in a very small range.

Initially this was occurring in Ubuntu in VMWare on my MBP, which given others issues with virtualisation and MFOC thought may have been the issue so I switched to using a Ubuntu Live USB on my MBP and the issue persisted.

What is likely the cause of the issues here? MFOC? The reader? The card or the laptop itself?