Automatic Transmission - 2002 Gmc Sierra Transmission Question

When I Put The Gear Selector In 1st Gear The Truck Will Still Shift To Second Sometimes. Is This Normal? And Why Does It Do This? Any Info Would Be Great. Thank You

I was doing a trans service on a 98 chevy 1500. When I test drive cars I allways put it in first. but this chevy went into 2 before I put it there. I guess that is normal.