Generally with an oil change you also change the filter. On most engines the filter holds a significant amount of oil and it will not fill up until the engine runs and pumps enough oil into it.
If you check the oil level before running the engine for a minute or so, you may leave the oil level low since you checked the level with the filter and also other ...
I have the valves on all my vehicles, c12 cat in motorhome, chev avalanche, two ram pickups. All are protected and work great. The unit in the motorhome allows me to drain a gal at a time which makes for a clean job.
Extracting the oil is perfectly acceptable. Certain cars ("Smart" if I recall correctly) don't even have an drain plug anymore. The residual amount of oil, assuming no mistakes during extraction, isn't relevant.
If you drain the oil, hot or cold, and wait 2 days for all the oil to drain out you still won’t get all of it out. Just take the engine apart and all the surfaces will still have a thin layer of oil on them.
So most of us just get the largest part out and then the ratio of new to old is high.
If you really want to make an effort then drain the old oil, fill ...