Hi, thanks for you answers, but forget all this... I was sure my system was up to date but it was not...  Somehow the Dell utility stop advising me since I don't know when...  So I run manually an update check, made all the updates, including a BIOS update, reboot, and now everything is normal, each tests report the same speed. It was my mistake, sorry for that.
Hi, I have a 120mbps connection.  When I run speed tests on all my personal devices connected on 5g canal (cells, tablet, xbox, personal laptop) it always report over 120mbps speed on every different speed tests. The speed test sites I use are : fast.com,  bing speed test, ookla, speedtest.net and google speed test. BUT!  On my Latitute laptop (for work) the first 2 (fast and bing) report a speed of only 12 or 15 mbps!  While the last 2 (ookla and google) report a correct speed over 120mbps... My drivers are up to date and I also reboot my laptop, but no avail...   Dozens of speed tests always report the same result! I don't understand why, someone has an explication?? Thanks.  
My Latitude report inconsistent wifi speed