It is possible because they are by design independently configurable. While not uncommon to use certain IP addresses and/or names for both sending and receiving mail, there is no necessity to do so.
If a certain address is found looking up mail exchanges for receiving mail, that alone does not tell you anything about whether that address will additionally be used for sending mail and should be designated as such in the relevant SPF record.
One thing worth double-checking, however, is any assumption that MX hosts are never sending mail. I often see both notifications ranging from worthless backscatter to somewhat-useful disposition notifications and out-of-office replies originating from hosts that their respective admin wrongly assumed as never sending mail.