Considering that there were some 12,731 B17s and 7377 Lancaster built, and that the B17 first flew in 1935 compared to the Lancs first flight in 1941, the B17 had many more years to get it operating correctly.
The B17 entered service in April 1938, before the war had begun The Lancaster entered service in Feb 42, not long after America joined in.
B17's dropped 640,000 tons compared to the Lancasters 608,612 tons, an average of 50 tons per B17 and 82.5 tons per Lancaster.