Response to agitation: I do it different; it works, so why is it the oddball way?

I recently replaced my 25 year old tank (which had no "twirler") with a nice new tank with a "Twirler" that not only rotates the reel but bounces it up and down a little. Very convenient. Negatives looked great until I developed a set of uniform gray card images (for zone calibration). They show decreased density in the center of the strip. I'm guessing this is due to the "twirl and bounce" agitation. Looks like I should go back to the old inversion technique. If I hadn't done the uniform exposure tests I probably wouldn't have noticed the effect for quite a long time - in fact I still can't really see it on "normal" negatives.

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