Using the simulations I compared the surface reference sample scan signal when changing some parameters.

First, I made the scan of the surface reference sample with the 633nm probe. The figure1 shows the signal AC/DC when I change the size of the detector.

Then I set 1mm of detector size and only change the wavelength and probe waist. Figure2

The interference a bit larger and the intensity is about half of the 633nm probe. This may be due to the different focusing alignment required for the 1310nm. However, it still doesn't explain the behavior of experimental data that have 2 large peaks instead of one.