Because whenever you log in to a website it can see the server string that includes all sorts of information about your pc, your operating system and your browser. It looks like this
109.152.159.2 – – [24/Jul/2018:09:26:44 -0700] “GET /someones-sending-email-address-stop/
HTTP/1.1” 200 47906 “https://www.google.com/” “Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_5) AppleWebKit/601.7.8 (KHTML, like Gecko) Version/9.1.3 Safari/537.86.7”
This shows that the person logging in is using google to search on an Apple Mac computer and the Apple operating system using the Safari browser