"Often" refers to frequency, "usually" refers to proportion.
Let's say I shop at nearby supermarket X several times a week, supermarket Y once a week, and fancy expensive supermarket Z once a month.
I rarely shop at Z.
My visits to Y are not rare, they are frequent. I often shop at Y.
However, I do most of my shopping at X. So, even though I often shop at Y, I usually shop at Z.