Envy Photo 7134: Printer detects wrong format

Hello community
When I load a 4x6 paper, the printer recognizes a 5x7 format. I place the paper at the front of the printer. Left and right it is centered by the paper guidances. Only when I push the 4x6 paper about 2cm backwards manually, the printer detects a 4x6 format.
But as I understood the instructions, the papier must touch the front guidances in all formats.
What am I doing wrong here? Thanks for your tips.