Penelope Keith and the stress pattern of English

In this week's Radio Times, the actress Penelope Keith gets worked up about the pronunciation of certain words: 

If I hear 'lamentable', she says with a shudder, 'or worse, 'irrevocable', I want to get a brick and throw it at the wireless. We have to keep screaming[...] because if we don't, this kind of this will become current.

Disregarding (or 'irregardless', if you prefer - it would undoubtedly annoy Penelope) the fact that it's already current, of she wouldn't be hearing it on 'the wireless', what's her problem?


Well, I don't have access to the OED here but Dictionary.com tells me that it is pronounced with the stress on the first syllable. In fact, Merriam-Webster gives it with the stress on the ment syllable, and stressed on the first as an alternative. Clearly, this is an old and well-established pronunciation. Still, Penelope Keith doesn't like it and others probably feel the same way. This is just yer basic peeving and not to be worried about.


But what interested me was that it actually seems odd to pronounce it the way she would like. English generally, in long words, puts the stress on the antepenultimate syllable:

an.te.pe.'nul.ti.mateex.tra.te.'rre.stri.al'fru.mi.ous 'ban.der.snatch

Not always, of course, there are exceptions:

ar.che.'ty.palpho.to.'gra.phic'ca.ter.pi.llar

I don't know enough about this kind of thing to know what causes these to be different, but I suspect it's something to do with the morphology and compounding involved in creating these words. But for lamentable and irrevocable, it seems absolutely natural to follow the pattern and stick the primary stress on the ment and voc bits, not least because we have la'ment and re'voke, although of course stress often changes when words are inflected (cf. pho.'to.gra.phy and pho.to.'gra.phic above). 


So why would we expect the pronunciation preferred by Penelope? I don't have an answer to this one; it's a genuine question. Answers on the back of a postcard (or in the comments).