The more creative the encounter is the more ways you have to adjust on the fly and the less CR is accurate. Sure it gives you an idea on which ballpark you are in.
For example you have multiple lair actions and a selection of those activate each round. If the PCs are hurting then you can choose a few soft ones to give then a chance to regroup and recover. If reinforcements are coming perhaps there are a few less. Similarly if the PCs are cruising, hit 'em hard.
Whether or not this crosses the border into DM fiat/fudging I'll leave up to you.
Also remember you don't have to use the highest DPR actions in general.