P-Sharan STD-35 and exposure

Hi there,
I built myself a P-Sharan STD-35 pinhole camera for a bit of fun http://www.sharan-camera.com/ but I can't quite figure out the suggested exposure times. The pinhole is stated as being 0.16mm to give F/130 for 20mm focal length which seems about right, but the suggested exposure time for ISO 100 film is 2-3 seconds for sunny conditions and 3-4 seconds for cloudy. Using the sunny F/16 rule, I would guess that F/130 should be about a half a second, so the recommended time should be about 2 stops over-exposed. The funny thing is, when I stick to the recommended times, they negatives are well exposed, but if I meter the scene and use F/130 shutter speeds (around 1/2 a second for bright sun) the negatives are underexposed. Any idea what might be going on?
Ta!

I have found most exposures on pinhole to be "cut and try". After standardizing on the film (or paper), the developer, etc I make a series of test exposures and from that derive correct exposures for future use.

Yeah, I don't understand why it doesn't conform to caculations either. Oh well.

reciprocity failure?

inaccurate size given for pinhole?

I have the same kit, and ran an expired roll of BW400CN through it for a first test run. I guessed most of the exposures, and most of the failures were underexposure, with only one completely blown out frame on the roll.

I tend towards overexposing negative film anyway, so just a bit more time than my normal guesstimate and it seems to be happy.