The answer to your question is "experience."
There's something nagging at me regarding my deadlift process, and an answer may have some limited utility for me in programming my other warmups. This is also a 'feelz' question, so beware.
It 'feelz' like there is a fairly consistent (inverted) effort bell curve at a new, higher deadlift weight which starts with max effort on the first pull, falling somewhat to the 3rd rep, and then climbing again to the fifth and final pull. I understand why pulls 3-5 get progressively harder, fatigue. I don't understand why the first three feel progressively, but only slightly, easier. Is there some 'small a adaptation' going on, where the musculature adjusts to a heavier load within that single set, or do I need to take a longer look at my warmup process to make sure I'm less taxed before I start the work set?
Even though it's obviously the same weight and requires the same amount of force to pull, should the set start 'feeling' heavy and end feeling heavy, should it feel progressively heavier 1-5, or is this just a stupid quibbling question?
The answer to your question is "experience."
You are making adjustments in efficiency. With a lot of work the goal is to be most efficient from the first pull.
I feel that this applies to all the basic barbell lifts, actually.
Obviously, you are not getting stronger between reps, but the differences you tend to feel are probably chalked up to your head rather than anything physical going on. You may expect the first rep to feel heavy, therefore it does. Once that is out of the way and you now know you can do it, you may perceive the weight to be lighter the next few reps before fatigue sets in. The limits upon human performance are not well understood and how your brain affects all of this is also not well understood. As a little personal anecdote, it is not unusual for my last deadlift warm up to feel very heavy. Often the first rep of my work set "feels" lighter than my last warmup. I have always attributed this to the fact that I know I need to pull multiple reps for my work set and approach it as such. I don't expect it to be so heavy that I cannot pull more reps, so it does not feel that way. The placebo effect is real and strong.
This also assumes you are deadlifting correctly, which may be dangerous.