OOP: My question about Petunia (slight spoiler)

[tish]

No: HPFGUIDX 63273

Here is a little problem that I found while reading OOP.  I am not 
sure if anyone has posted this before because I have been unable to 
go through each message yet.

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Petunia says that she heard Lily talking to James about the 
Dementors being the guards of Azkaban, but how can that be?  Didn't 
DD say at the end of GOF that the Dementors are natural allies of 
Voldemort??  Wouldn't that mean that they were NOT the guards of 
Azkaban when Voldemort was in power?  If they have rejoined thier 
side now that Voldemort is back, wouldn't that indicate that they 
were on his side before the downfall?  Now, if my assumptions are 
correct, there is NO WAY the Dementors were guards when James and 
Lily were in school.  Even if they were the guards before the reign 
of Voldemort and went over to his side after he was stong(negating 
the "natural ally" comment), I can't see the MOM putting them in 
charge of Azkaban knowing how easliy they flip sides.

Did anyone else find Petunia's knowledge of them VERY fishy?  

Tish
(Who, despite her best efforts, is really starting to care about 
what happens to Snape)