See, my big number days come on top, almost by definition.
I'll agree nymphing is more consistent. If nothings going on, I can very likely pull out a few from underneath.
But in a hatch situation, I get some time to fish the nymphs before it starts, and pull out a few that way. Then they start breaking the surface and I go on top and have a field day, thats when I get the #'s. Then it dies off. The # of fish I catch is strongly related to the duration of the topwater event. My only hope of, say, 15+ fish is a long duration of topwater action. Aside from brookie fishing, all of my big number days were situations where a hatch or spinner fall lasted a long time. I hit 30 fish days frequently with brookies, and sometimes more, and thats usually 95% on dries as well.
But when I'm on the stream, there's a hatch going on a high percentage of the time from say March through September. A hatch isn't a rare situation at all, for me its more common to be fishing in a hatch situation than not. I agree I self select that a bit, but its not hard to do and doesn't take any great effort. You simply think, ok, where locally am I gonna find something happening? And you go there!
In March I'll hit up streams with BWO hatches. Come April I'll switch to streams like Penns and LJR for grannoms, or else early season stockies. In May, everywhere is full of hatches nearly every day all day, you got the March Browns, sulfurs, several varieties of caddis, etc, barring severe weather events you can't find a stream or time where something isn't happening! In June, you got drakes, more sulfur varieties, still some caddis, etc. From July through Sept., I often switch to brookie streams, throwin Adams, stimulators, humpies, wulffs and such. When I do fish bigger waters, its in the morning chasing tricos or the evening skittering caddis for the egg-layers, some terrestrials, and the occasional swarm of something random. After that winds down, the BWO's are once again on.
I mean, really, you can fish every day from mid-March through mid-October and find a local stream where you can rightly expect topwater action. There are times where you're wrong, and yes, I nymph during these times, but they are never my best days.