The second query is a red herring. It will do this:
SELECT @Pist = dbo.udfOrGetDetail (@OID,@OTID) +'<br />'
as many times as rows returned by everything after the FROM list. Each time it runs it will generate exactly the same result, and then it will throw away all except the last result. As Kevin has pointed out, there is some scope for performance improvement here.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden