Those vegetarians who avoid garlic, onions, spring onions, leeks, etc. are probably doing it for religious reasons as such items are said to encourage emotions like anger, turmoil, etc. Those vegetarians who avoid eggs & diary products are known as vegans, if I remember correctly.
I have met several "ang mohs" and some locals who are vegetarians. Later I discovered they are "seafood vegetarian", "fish vegetarians", etc. which are strictly not vegetarianism. It's just that they avoid meat, or sometimes, red meat (so they could eat chicken :dunno: ).
The most ridiculous case I have come across was an ex-colleague who proudly claims that she's a vegetarian, but she would share other ppl's food, e.g. pick on other's char kueh teow or laksa, as long as she doesn't eat the cockles, the sausages (the meat), so that she gets to eat and taste the same food as non-vegetarians.