Bluntly put Madina with 2400gp is barely able to rebuild our armies quickly, whereas Aurvandil has bested our military strength two or three times with only 1600gp income.
Or, there are Madinans wasting gold by hording, buffing family accounts, or some other use.
Or, Aurvandil has found a few units with optimal gold/CS ratios and repeatedly uses those RCs.
Or, they recruit more, smaller units rather than fewer, larger units, thereby optimizing CS.
The idea that realm income will necessary translate into CS is just silly. Plus, the income listed IG is an estimate. They could be running higher average tax rates than Madina, so their income may be understated.