PMThroughput accounting question

Good morning, There is a question regarding throughput accounting ratios where a three step process is described, Assembly, Quality Control & Packaging [ACCA PM CH.4, CBF Throughput Accounting]. It is then deduced that Packaging is the scarce resource, however, all the calculations in the solution are based from the QC figures? Not sure if I am missing a crucial point here but whenever there was a previous scenario like this, the only figures used for calculation were the Bottleneck stage as well, and not the stage preceding the Bottleneck? Thank you kindly, Kabir