Depot contraceptive Injection

I've been on the depot since sept 14' - sept 15'. Was told I probably wouldn't see a period for a least year. I had my first period in the Dec 15' and since have been having periods every 21 - 23 days, when my usual cycle is every 26 days. 
Anyone have any ideas as to why this is happening? The periods are normal, no lighter or heavier. 
Just surprised to see them I suppose, and unsure as to why they happening so often.