A macro lens like the Tamron is designed to work from macro distances to infinity in one range. Some models may have a focus limiter switch, to save the AF from seeking the whole range if not needed. The focusing range from end to end tends to take longer on macro lenses than others.
I haven't used the G12 specifically, but with experience of other compacts which also have a macro mode, I think something similar is happening. In normal day to day use, the focus may be slow to go from one extreme to the other. So to work around that, they can put in a limiter to do one or the other. If you know roughly what range you're working in, setting the camera to that mode can speed things up a bit.
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.