Quoting from Neil Hodgson from another report:
SciTE tries to avoid creating multiple empty buffers. If there is any text in the first tab then New will create another buffer. Fixing this could lead to unwanted buffers and the problem is very limited in scope so I won't be fixing it.
If the first empty buffer has text in it it is possible to create multiple buffers. Relying to the quote SciTE should limit this to 1 buffer.