sdcch traffic is about 10 times smaller than tch traffic (approx.)
unsuccessful calls (mean there was a failure during the call setup) do not use a TCH carrier. The failure occurs while they are on the sdcch channel. therefore they simply cannot be counted in the tch traffic.
if the failure occurs after the callsetup, then it is called a call drop. A call which is dropped occupies a tch carrier for a while, therefore it is counted in the tch erlang.