Facebook says this person has four mutual friends with me but only three appear.?

[Mathew]

Basically doing some looking around on Facebook and it says that this person has four mutual friends with me.
So I click to see who they are but only three appear. What does it mean? Four mutual friends or three?

[Babyface]

One of them is a ghost. Ghosts count as friends on Facebook, but for obvious reasons they do not show up.

[george]

The chances are 1 of them has blocked you, no hard feelings.

[EliteBob]

it could be that your blocked by one of there, or just maybe one of them deactivated their account?

[Wendy claire]

maybe the fourth one blocked you

[Sadia]

i think 4th of block you.