Yes what you observed is normal and how the programme works.
When you hit a new tier, the amount of XP for that tier is deducted. Your qualification période also resets. So now you have another 12 months to attain 180XP to reach Gold. Once you reach 180XP in the new qualification période, you then obtain Gold and 180XP are deducted; you now have another year to obtain 300XP for Platinum.