I'm inconsistent
I've shot rounds which started well and tailed off, some which started poorly and improved, others which stayed pretty much the same (good or bad) all the way through.
The only time I've seen marked "dips" which I could actually attribute to something (other than that old fallback "fatigue"), it's been -
Equipment failure (rest on it's way out, sight block coming loose, stabilisers falling off...),
External interference ("you're doing rather well today, aren't you?"
).