Extension tubes - focus distance oddity?

My new extension tubes arrived, so I popped them on to my various lenses to see what working distance I will have. I was expecting minimum focusing distance to decrease until I was unable to focus, and maximum focusing distance to also decrease.

15-85mm at various focal lengths - no surprises.
30mm - no surprises.

But the 100mm macro - whoa. The infinity focus reduced to a minimum of 42cm with 36+20+12mm tubes (so far as expected), but the minimum focusing distance actually increased by a couple of cm with increasing tube length, from 30cm to 33cm.

I'm guessing that this is some result of the particular design of a macro lens, but I'm interested if anyone can explain why this is the case.

I can only assume you made a mistake somewhere. With all the tubes on and at min focus then the magnification should be about 2:1 and that can only come from focusing closer.
Brian v.

Well, closest focus was closer to the end of the lens, while the minimum focussing distance (distance to sensor) increased. The working distance decreased from about 13 cm (no tubes) to about 10cm (68cm tubes).

Perhaps magnification is a factor of focus distance from the lens elements, rather than from the sensor.

Given that the bare lens goes from 1:2 to 1:1 between about 34cm and 30cm focussing distance (4cm for double magnification), it doesn't seem unbelievable that a further 3cm reduction could double the magnification again.

This behaviour is obviously quite different to my non-macro lenses, however, hence my post.

The distance from sensor is shortest at 1:1.

The distance is the same at 1:2 and 2:1, the only difference is that the lens is nearest the sensor at 1:2, and nearest the object at 2:1. Try with a reversed lens, and you will understand it has to be this way.