[Qgis-user] Map composer. Wrong map scale

[Carlo Cormio]

Dear all,

I'm using QGIS 2.12.3 on Windows 10. After printing a map using QGIS map 
composer, I checked some distances and found that map scale was 
different from the expected one.
I set ISO A3, portrait, as page size. QGIS project has EPSG:3857 (due to 
the use of a google satellite layer), but most of the other layers have 
EPSG:32633. I tell you this in case it could affect prints.
Map scale in print composer was set to 1:5000, but when I measured 
distances (I also put a scale in the composition) I found it was more or 
less 1:3500.
The strange thing is that the ratio between the two scales is 1,41, that 
is exactly ISO A3:A4 ratio.
What did I do wrong? Is there a setting I missed?

Hope I gave all the necessary information.

Regards,

Carlo