perhaps another reason that seed compost is made with low nutrient levels is so the manufacturer doesn't have to purchase the nutrients to add?
Nah, the cost of adding a 7-7-7 fertiliser to get the EC to 500 in a retail multipurpose compost mix is here nor there relatively speaking. Remember that many of the compost makers dont buy fertiliser, they buy raw materials (like urea and rock phosphate) and make there own.
On the other hand the cost of compensating a grower for 20,000 african violets because the seed compost you supplied to the grower was too hot and fried the seedlings would run well into 5 digits. Only from that point of view is it cheaper not to put it in the mix !