Strange RADIUS statistics behaviour?

Hello. Please explain me, may be I do not understand something, but I think that there is some strange behaviour in RADIUS statistic counters.
I’m using IAS RADIUS server and RB411AR with RouterOS 6.4 to authenticate my domain users on wireless connection with EAP-MSCHAPv2 passthrough.
It results in some dialogue between IAS and caller consisting of several Access-Requests / Access-Challenges and Access-Accept (or -Reject) in the end. But it is strange that RouterOS counts all Access-Challenges as Rejects in counters. You can see attached screenshot for details.
So, is it normal (but some illogical) behaviour or may be it is a kind of a bug?
Thanks.
radius_why.jpg