Why does it say somebody is still following me even after they’ve blocked me?

I was checking my followers on another account out of boredom and saw that it said somebody that had blocked me back in October is still following me. I checked to see if they had me blocked and it said they did, so I don’t understand why it says they’re following me whenever somebody checks my followers. Is this just a glitch or something? Because it doesn’t really make sense if this glitch is lasting for so long.