The Tibetan consonant inventory has no f, no v, and no interdental fricatives, so the sounds that dominate English and many other languages have to be approximated when a foreign name is written in Tibetan letters.
The commonest solution is to use the nearest available letter, which for f is pha, the aspirated labial. That is why transliterations of Tibetan renderings of foreign words look odd in roman script twice over: the sound has been approximated once going in, and the approximation has then been transliterated back out letter by letter.
The script does have machinery for foreign sounds, but it was built for a different foreign language. The reversed letters and the subjoined ha were devised to write Sanskrit precisely, because Buddhist translation demanded precision in names and mantras, and they solve Sanskrit problems and not English ones. Nothing equivalent was ever standardised for European languages, so modern practice varies: writers and publishers make their own choices, and the same foreign name may appear in more than one spelling.
There is a wider point in this. A script records the sound system of the language it was designed for, and it records the sound system of any other language only as far as someone has bothered to extend it. Tibetan extended itself for Sanskrit with great care, and for everything else, so far, chiefly by improvisation.