Hi,
First please understand, queueing config is only applied, if congestion exists. So class-default traffic could take 100% of your interface bandwidth, IF no other traffic is present.
Second, in most cases there is also traffic not accounted for in the class-maps, f.e. L2 keepalives, system packets, CDP, and so on. The router will prioritize some of this traffic. Therefore allocating 100% to user traffic is not feasible and thus the (conservative) approach with 75% of interface bandwidth.
Per se it is therefore also not simple to say X% of traffic is allocated to class-default.
By the way, the QoS 2.2 class covers this in detail.
Regards, Martin