I'm not sure that is the case, although I haven't tried it. But don't think there's much difference in recurve stability with a sidenocked bow or a victorian-style nock. If any difference, I would think the sidenock would be more stable if it was combined with a self tightening knot. If the recurve is bending enough, a narrow-tipped longbow will have stability-issues either way. And you usually need to adjust the recurves with heat to make them stable anyway.
My bet is that this is something they did to their bow when it had been thouroughly used, and they wanted to speed it up a little. A longbow is so long that there is little help in recurves, but recurving a longbow with a lot of stringfollow will make it simulate less stringfollow.
Simon: take a look in Hardy's Longbow, where one of the original bows is shown on the tiller. That would probably be as close as we will get to see how one of them were tillered.
When I see "replicas", I always wonder how close they are to the original measurements. Up till now, I have not even seen one bow that was made stricktly from measurements. If you guys know of any bow like this, let me know! Just following the general width of the originals will not do the job in seeing how they were tillered. Sure, we can make them stress the wood as evenly as possible, but that may be different from what they really were tillered like.