Transtioning to a so called complex aircraft you are also suposed to do a differences training. That is you fly a C172, then you want to fly something which has retractable landing gear and/or variable pitch prop.
Your differences training from whatever you flew to the C172 was probably not required by the authorities as RTN11 points out, it is probably a requirement of the flying club you are renting the aircraft from. That is why the FI said you can log them as PIC.
If it was a CAA requirement, then the FI would have to be PIC, and it would be endorsed on your logbook as differences training.
you will need a differences training in all of the following cases
-taildrager to tricicle undercarriage or the oposite way
-Fixed pich prop to variable pitch prop.
-Fixed landig gear to retractable landing gear
As far as I know there is no requirement for a given amount of training for any of the above cases, just the amount the instructor decides you need.
Then it gets a bit tricky when it comes to decide if you need or not differences traiing when transitioning from say a DA40 to a clasic variable pitch prop like a Piper archer, because the DA40 has a constant speed prop but it is handled by a computer, that is you do not have direct control over it, while a clasical variable pitch arrancement you do the changes manually with a lever.
I do not know if there is any rule regarding this last case, although common sense dictates one would need the differences training.
Have nice flights
LEVC