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The way I read this, is that the boat loses 1 ton of buoyancy by diving to considerable depth (150-200 meters?). It is impossible to say what amount of this is caused by leaking and what amount by hull contraction. I would assume most of it is due to hull contraction, because even 1/10 ton is 100 liters of water, which sounds like a huge amount to just leak in.
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I just double checked with the original german version and the translation doesn't seem to get the correct meaning IMHO. I think that what it means is that you should have up to 1 ton of positive buoyancy when deep diving, i.e. the boat should be trimmed to weight 1 ton less than what would sink it. In a 780 tons boat this is a tiny figure, but enough to ensure that dive planes keep the boat down and take account of leaks, etc. I think that BDU probably also considered that the boat would take in water if depth charged, so preferred that captains had reserve positive buoyancy in any case. Remember that BDU also wanted them to surface as quick as possible and engage again.