Phil Yes Phil, let's do it!
Though Tim elaborated a bit on it, it is fascinating, these differences between our Countries/Cultures.
Once upon a time, during my years at the Academy as a student --- no I am kidding: In a 'nutshell': I needed to model a small IT system with my German friend and study mate. We needed some basic information about a person and it took me a full hour to explain to him the Dutch naming system and yes HOW a first initial can deviate from one's first name. It was so funny! I think we created a list on funny Dutch or German weirdness.
Anyhow, your name is Phil which is not suitable for my example (I really tried :'|) so let take "Johannes" who is also in this topic. Within the name 'Johannes' is another name hidden, 'Hans' - Yes I know, is not literately there, but there is a relation between two names. I believe the word would be heritage or deviated from.
So though Johannes might have this name listed in his passport, his family might call him 'Hans'. Some of his friends might have even no idea his passport name lists "Johannes".
Johannes' father might bear the name: Antonius (and but might be called Anton). As tradition continues our Johannes bears his father's first name as his second name.
So our "Hans" is listed in his passport as Johannes Antonius Smidt. (for example.) As others have mentioned, more names could be added; a grandfather's name from both sides of the family etcetra, but let's not blow it just yet ;). - So, Hans' initials are formally "J.A." [Smidt]. But everybody could be calling him Hans.... or Johan.... - Hmz, that is a better fit, but then the intial would be the same so the example wouldn't match so please let that one slide because this does occur.
Before we end this Excursion, as Phil requested, I have seen another variation where intials and first name do not match.
Simply because the (legal) "Call Sign" (horrible translation but I also cannot find a better one) does not match the first name.
The example above with the difference what Hans is called and his real first name might be caused due to some baptismal name heritage on which a person chooses a slightly different name to be called (or his parents do during registration.)
Sometimes there is no baptismal name and the parents simply gave the child multiple names. Usually the call sign is equal to the first name but that is not always the case, so you get:
Rob [Robert Johannes Geradeus] van Aken : R.J.G. van Aken.
Renee [Maria Axxx] van Zandvoort : M.A. van Zandvoort.
Johannes Antonius Smidt: J.A. Smith
Hans [Johannes B...] Oosterveen : J.B. Oosterveen.
[Excursion inception]
Before I leave you to absorb this excursion, on the top of our Craziness list, is not this name weirdness... it is the fact that his grandparents were amazed when I congratulated them with their grandson's birthday (handshake & saying congrats). Apparently it is very Dutch to also congratulate the family of the birthday person ;). Even his girlfriend thought is it was weird I congratulated her. Obviously I have been doing it every since. "The Dutch friend!"
[/Excursion inception]
P.s. @ topic-starter: it would make sense that the name that is written on your credit card is also used when making a payment in the system.
(One can choose your name variation when ordering a credit card, I am assuming based on your post that at the moment, they are unrelated. #Extension Point.)
@Phil: I hope you enjoyed it, I had a blast writing it; good memories ;)
Have a great weekend everyone.