Almost certainly key-bounce. The IBM design is completely immune to it. I'm a little surprised that there are still keyboards that have bounce.

It's probably not your finger bouncing, but the keyswitch. You might be able to prevent it by typing slower and more rhythmically (the way my summer-school typing teacher taught us to, back in the days when high school typing was taught on Underwood and Smith-Corona manuals).

There's a reason why I have almost a dozen surplus IBM keyboards in my garage! They don't make 'em like they used to.