Internet Speed

Someone please give my a logical explanation for this.  I had until today a 2GB Internet plan.  I was getting download speeds in the 900Mbps range up until about a month ago. Then for whatever reason it slowed down to between 500 and 600Mbps.  Since my router and computer are only able to reach 1GB I decided to call and see what other plans were available.  So In switched to a 1GB plan.  They refreshed my router I assume to activate the new plan and now I back to getting over 900 Mbps download.