I'm not exactly a muslim by traditional definition since as a quranist I'm considered a heretic by the majority, but allow me to try answering your question.
There multiple sects within Islam.
This is the truth, but it's so inconvenient for muslim to admit that they are actually following a sect which all claim to be following God's command, that they would not admit they're following certain sect (unlike what you've seen with the Christians).
Traditionally, based on the doctrines accepted by majority of muslims, there are 4 source of Islamic jurisprudence:
- Ijma 'Ulama (consensus of those they considered knowledgable)
- Ijtihad (deciding the rule for new things based on parallel or analogy, e.g. the rule for organ donation, e-commerce, etc.)
Based on the interpretation of those above, there would be a different set of laws implemented on different places.
Countries like Somalia, Nigeria, Egypt, Saudi Arabia, Bahrain, Iran, Uzbekistan, Malaysia, Brunei etc are all claiming of implementing the sharia law, even though they have different interpretations.
Thus I think come the question "which one" when somebody's referring to sharia.
In fact, that's the same reply that I used when debating with someone about sharia implementation