As I wrote above, I think that, in
American English, it's standard to add a period (alt. full stop) after any abbreviated title. Thus if you have always been in contact with the English language as used in America, it's may not be really surprising that you did not hear about the difference.
What I learned (in French, but I imagine the justification must be the same in British English) is that the full stop / period actually stands for the part of the word that is omitted; thus if the part of the word that is omitted is in the middle, it makes no sense to add a full stop / period at its end.
British English guidelines by a translation specialist (see under "Abbreviations and contractions") :
Our in-house style guide for linguists translating into UK English.
stptrans.com
"St Helena" as written on St Helena :
Here you'll find accommodation, flights and information and things to do when visiting St Helena
www.sthelenatourism.com