An extended type can change variable types?

This kind of makes sense in a way I can't quite put my finger on...

Strict

Type MainTypeData
	Field name:String="MainType"
End Type

Type ChildTypeData
	Field name:String="ChildType"
	Field iamdifferent:String="I am different!"
End Type

Type MainType
	Field data:MainTypeData=New MainTypeData
	
	Field bleh:Int=4
End Type

Type ChildType Extends MainType
	Field data:ChildTypeData=New ChildTypeData
	
	Field bleh:Float=4.51
End Type

Local Main:MainType=New MainType
Local Child:ChildType=New ChildType

WriteStdout(Main.data.name+" bleh is ")
WriteStdout(Main.bleh+"~n")
WriteStdout(Child.data.name+" bleh is ")
WriteStdout(Child.bleh+"~n")

End

What I think I'm doing here is giving an overrided type a different return type than its parent. This is good (excellent for my purposes!) but kind of surprised me.
Why can I override the type of a variable by extending, but not the return type of a method?

With this in mind, I am probably very confused about a lot of things...