Yves, I've got Elsa's Schengen Visa here with me now, and its says:
Estados Schengen
Valid from: 11-07-06 to 10-01-07
Type: C
Numero De Entradas: Multi
Duration of Stay: 90 Days
Now, maybe this is where the confusion lies; you are correct the Visa is ok for 90 days at each trip. But is valid for 6 months, meaning you could make as many trips as you like, as long as each trip stays within 90 day limit - is that what you are referring to?
I remember when we applied for the Visa, we asked for a 6 month multi-entry visa, and we were told, no problem, and we got it.
We used the visa twice; August and October, and whats more annoying neither UK or Spanish Immigration actually looked at it. IN fact, on our return flight back to the UK, they took her passport away at Alicante to get a photocopy of her UK ILR Visa, they weren't interested in the Schengen Visa, even though we pointed it out to them.
I did some googling yesterday, and it seems not all countries in Schengen pact offer 6 monthly validity on their Schengen visas.
http://www.theschengenoffice.com/spain_embassy.html