How to interpret Tukey's b

#1
I was wondering if anyone could assist me on how to interpret my Tukey's b results. I need to indicate which clusters differ significantly from each-other with a letter. My problem is that the Tukey's b seperates my clusters 3 times, so you would expect that there are three significant differences (i.e. a,b,c) but I only detect two.

I would appreciate it if somebody could help me interpret it.

Thanks in advance!

Pieter
ps. tukeys results attached below in word format