You could be 'not a typical Leo' for all sorts of reasons - mostly because the Sun sign is only one of the placements that goes to make up your character - there are nine others, plus Ascendant and MC (and this is the short list!).
My guess is that whoever told you this is working on the presumption that 'Leo' is a highly 'theatrical' sign - wanting to be the centre of the stage. Leo is also often interpreted to mean self-centred, proud and also generous. It's a magisterial sign.
Now the twelfth House rules the hidden. Modern Astrologers will make claims that it is the house that shows how you want to merge yourself with something greater (not a Leo trait). A more traditional interpretation would link it to solitude, institutions such as prisons and hospitals and also secret enemies and 'self undoing'. None of these are seen by the modern Astrologer as 'Leo'
The clash therefore is between Leo, wanting to be centre stage and a house which suggests that he or she can't be centre stage.
However a word of caution. As I said at the outset, you are much more than just a Sun sign and even if your Leo Sun was on the MC, shining for all to see, you might still not be well described as 'Leo'. As I said in another thread, reducing people to Sun signs is at best an oversimplification and at worst can be completely misleading.