Hahaha! "Unusual". What an interesting understatement to describe it!
Lots of ways for a system to supply pilots.
In JAR-land, ATPL exam content has many common areas with CPL + its own extra bits. Similar to how CPL exams cover all stuff for PPL level knowledge.
In other parts of the world they're different knowledge areas. In those cases, exams for the higher licence won't cover you for the lower level ie ATPL exams passes don't also cover you for CPL exams. In the US & Oz CPL exam(s) are completely separate to ATPL exam(s).
FAA has a separate exam + a flight test for each licence. An IR doesn't lapse (has recency requirements but it still exists). ATPL includes IR privileges. The ATP flight test is essentially an IR flight test with tighter tolerances.
Oz has one or more exams & flight tests for each licence except no flight test for the ATPL. IR lapses. ATPL requires an IR for IFR ops (NB IR also needed when applying for initial issue of ATPL)
JAR has lots of exams + a flight test for every licence except no flight test for the ATPL. IR lapses. (NB IR also needed when applying for initial issue of ATPL).
In the Oz & JAR models, an applicant for an ATPL doesn't need a separate flight test since s/he will have done one within the last year (IR issue OR renewal). In the US the applicant has to do an ATP flight test because s/he may not have had any form of rating/licence test since their IR or CPL issue.