Tonta, I thought your original post was interesting, covering a word, usage I hadn't really thought about. I've just looked online and from the Oxford dictionaries I was given several examples of the usage you don't like -
The number of priority homeless cases has reduced slightly.
Chambers also gives 'verb intransitive' for 'reduce', followed by a definition of 'to become smaller or less'. That seems to validate the 'numbers have reduced' you mentioned.
PS I don't know a great deal about the technicalities of grammar, verbs etc, I just like looking in dictionaries!