I wonder what causes this type of bimodal "quality of work" distribution.

It might just perceptual: it might be the case that days very into on of the two types are easier to recognize, tag and remember, whereas mixed days are not so vividly remembered.

I surely stop after a day of lots of little tasks and say "phew! What a day!" or after a big task is done "yeah! look at this cool feature you built today!" And even play with it a little. Normal days are not so reflected upon...

It's a result of many unconscious decisions by your brain which aim for regeneration and conservation of energy mainly.

Even when those are not needed..., the brain isn't too clever about modern work-load.

Very good question, I thought many times about this and could think that probably the beginning or how you start the day determines the rest of the day.