Here's an overview of the privacy issues with BIP37.
If you create a bloom filter and pack it with a bunch of addresses, it's not really possible for a third party to know what addresses are in the filter. At best they can say an address is in the filter with a given probability.
But everything changes if you create two (or more) separate filters with different sizes and/or fp rates containing the same addresses. Now the probability that a transaction would match both filters and still be a false positive is the false positive rate of filter A * the false positive rate of filter B (and filter C, D, E, etc). So in otherwords if someone creates more than one filter you can say with near certainty which addresses are in their filter.