some of the sp's think that the old craigslist rules apply to the erslist site, and they write the words for the numbers out like they used to do on craigslist, or they put spaces between them or asterisks or what have you and that stops the search engine from being able to pick up the numbers.
one assumes this is just ignorance and not a deliberate attempt to limit searches because they don't offer good service - but one should be on their guard when the seach doesn't work for some reason