The BTS sends Immediate Assign Reject when the Channel Request cannot be followed by a SDCCH allocation. (so far, we agree)
In this message, the BTS tells the MS to re-send a 2nd Channel Request only after xxx seconds (timers names are WI_OC, WI_EC, WI_xx)
From MS point of view : instead of sending the 2nd Ch. Req immmediately, it will wait accordingly to the WI_xx timer.
The 2nd Ch. Req has more chance to be succesful because after WI_xx seconds, one SDCCH is perhaps available. Indeed, the SDCCH usage is kind of bursty, and waiting xxx seconds is better than re-attempting immediately.
It is up to you to interpret this mechanism, and its impact on the indicators. I don’t know your RACH failure formula…