Strange bug in report

Hi to all, running 7.9.11 and i have a strnage bug on some report.
i’m reporting on project. I group by in my custom field “tipo_mandato_c” and i made a sum of the value “mandato”. tipo_mandato_c is a dropown with 4 field:
empty
Contratto singolo
contratto continuativo
studio fattibilità
But when i made the report, it divided the type for some reason, and i still don’t understand why:

But if i do another report similar to the first, with a group by on tipo_contratto_c and a count on the id it give me the right number of groups:

any idea of why it’s happening?