The reaction the goes into producing LSD in a laboratory is very complicated - There are several different processes and all can be manipulated or modified according to the particular chemist running them, depending on what precursors or equipment he may have at his disposal. And as much as we wish that every tab was laid using only optically pure 99.99% GCMS standardised Sandoz lysergic acid diethylamide tartrate... They ain't.
Crystal LSD commonly ranges in quality from 60 (less commonly these days) to the high nineties - The real clean fluff is usually reserved for inner circles, but nonetheless you can still tell the difference between higher and lower purity acid.
Why? Well, if you have 150ug of 75% pure LSD crystal, then you have 37.5ug of god knows what else. Usually it's reaction impurities. Depending on what those impurities are, it can subtly change the nature of the trip, or have no effect whatsoever. It could be 37.5ug of some other lysergic amine analogue produced in the reaction that wasn't removed during the cleaning step, or it could be a few specks of sodium carbonate left from another stage. not only that, if you know your crystal is only 75% pure, when you lay the sheets, and you're going to lay it at a concentration so that you wind up with 150ug of actual LSD on each tab then you're going to need 200ug of actual crystal, meaning there is therfore 50ug of impurities.
I've had plenty of blisteringly clean acid that just lit up my mind and spine and left every organ in my body singing, and i've also had some pretty gakky stuff that was visually stronger but also had an unpleasant body load (e.g nausea, cramps, strange aches, bruxism). I've had acid wherin, some batches always seem spiritually quite 'empty' but visually fulfilling, wheras others gave less fractals and more thoughts - A lot of the timne this comes down to set and setting, but there's often consistence.