[Question] Are all band sizes created equal?

I just took my measurements and the calculator suggests that I'm a 38D/38DD. Currently I'm wearing a 40DD and my band is digging in me. I have it as loose as possible. My cups have gaps in top, so I'm absolutely open to a different cup size, but the band suggestion has me worried.

This is my first time posting, so if I'm not doing this right, I apologize. I've measured myself before stumbling upon this sub, and I'm consistently a 38D/DD measured I'm in the US and now suspicious that my bra band width is lying to me.