Does anybody know why Apple uses two horizontal lines for hamburger menu vs three horizontal lines?
I think if using 2 lines will make the transformation into "X" more make sense than using 3 lines, which is in IOS it self 3 lines has a meaning already.
I don't understand where the question arises from.
Apple technically doesn't have any guidelines for Hamburger menus. If you check out the iOS Human Interface Guidelines you won't find any guidelines for hamburger menus because iOS apps based on the guidelines don't have any.
If somehow Apple were to use a Hamburger menu, it would be in the Navigation Bar, but of course, there isn't any. What Apple uses instead for navigation is called a Tab Bar (something akin to Bottom Navigation on Android) for quick navigation among the various screens. Also, Apple recommends using only 3-5 navigation item in the Tab Bar.
Avoid having too many tabs. Each additional tab reduces the tappable area for selecting a tab and increases the complexity of your app, making it harder to locate information. Although a More tab can display extra tabs, this requires additional taps and is a poor use of space. Include essential tabs only, and use the minimum tabs necessary for your information hierarchy. Too few tabs can be a problem too, as it can make your interface feel disconnected. In general, use between three and five tabs on iPhone. A few more are acceptable on iPad.
If you yet wish to present more items in your navigation, use the More icon on the list of Apple's System Icons and create a separate view with the list of additional items in their order of utility.
On the other hand, the icon like a Hamburger Menu you mention, on Apple's list of System Icons, is used for representing Most Viewed items, and even that uses three horizontal bars following three dots enclosed in a rounded rectangle, placed one below the other, just like the hamburger menu icon.
Hence, I believe, the premise of your question is false in itself.