The detailed example is consistent with previous discussions. The ACD reports service levels based on the ratio between calls offered and the call answered with the target service level (service level threshold above, and as 90 seconds in the example).
The reason Service Level and Answer Rate, as defined above, coincide is not a mere coincidence. Experienced call center planners set their Service Level targets in accordance with actual traffic and avaialble resources, so that under most operation conditions they meet the Service Level target. Consequently, their Answer Rate will always be only slightly better than the Service Level, accounting for a relatively small number of calls answered outside the target service level less abandoned calls.
As noted previously, some ACDs allow the definition of slightly different service level calculations.
I am not sure I followed the last two paragraphs. Service level is simply defined as percentage of calls answered within a predefined time, e.g. 85% of the calls will be answered within 90 seconds. The call center staffing is designed to sustain this performance level independent of the number of calls offered at any period of time (but, obviously staffing level is calculated for a specific average call volume). Therefore, to report servie level we need to use percentage, not absolute numbers. The reason you can guarantee service level in a well-designed call center is because you leverage the statistical nature of the calls queued by the ACD (see
http://www.diagnosticstrategies.com/...20Modeling.pdf)