Yay! Time to celebrate!

It most definitely means your heart is becoming more conditioned and fit. Your heart rate is lower because your heart doesn't have to work so hard to get oxygen to all your tissues -- and that's a very good thing.
But you're right that your body isn't working as hard as it had been as a result of your heart becoming more efficient and strong. If you want to keep your heart rate as high as it used to be, you're going to have to work harder. Meaning more resistance or faster speed on the elliptical.
Rather than changing speed and resistance at six week intervals, why not just aim for a certain HR (like on your peak intervals) and set the machine on whatever it takes to get you there? That's what I do and yep, I have to keep pushing the resistance up, up, up. But I'm OK with it because a cardiologist told me last year that my heart is in better shape than 99% of people my age.
You're doing great, Kim!!