The actual reasoning behind getting diminished returns over time is because of getting closer to your actual genetic potential yielding less gains with every cycle. That's why doses get larger and more compounds are added.
Most people are well under theur genetic potential when they start their first cycle which is why you can add an easy 20 lbs. Once you become advanced and have a large amount of muscle mass you're not going to add 20 lbs of muscle. You might be lucky to get 5. If we could all continue at the same rate everyone would be 300 lb ripped bodybuilders in a few years
(PM me for a price list for Biotech Labs and 10% discount)