I’ve seen this happen myself. I have even seen ISWC have the same work listed more than once when BMI/ASCAP does not.
There is no one simple answer because there are different reasons why we may get different results.
To answer the first question, I find BMI and ASCAP easier to work with - user friendlier. So that is where I would go. Especially considering that they are the originators of the data.
The second question is, again, one of those different reasons/results kind of questions.
In general, this doesn’t happen with ISWC. But if you go to BMI/ASCAP and do a broad name search, you will often times see the same person under different names, each name having its own number. The only way to know that they are the same person is to open them and see the same song list.
In fact, just a few hours ago I was at BMI and did a songwriter search for Bonnie Bishop. Five results displayed, but they ended up being 2 people.
|1.|BISHOP BONNIE|406258472|
|2.|BISHOP BONNIE E|406258374|
|3.|BISHOP BONNIE ELIZABETH|406258276|
|4.|BISHOP BONNIE LOU H|227826162|
|5.|BISHOP BONNIE LOU HAZEL|227826064|