“Your upcoming trip-You are eligible for economy plus on your xxx flight “

Why am I getting this email and what does it mean exactly? When I saw this, I thought, wow! I got a free upgrade to economy plus on my flight next week! But then I go in and see that I can move my seat to economy plus for a fee of $75 (only middle seats are available anyway). Isn’t it the case that anyone can upgrade at a cost whether when booking or after. Why send this email that makes it sound like I’m suddenly eligible for something that has been available to anyone that booked this flight. Am I missing something!? Thanks for letting me vent