An Hour Late All The Time?

well it takes me approximatly 10 min or so to get to my girlfriends house, i texted her that i was on my way at 215 pm and i asked her if she was done getting ready for the day she says no.then im on my way out the door my sister pulls into my driveway with my nephew and i got to blabbing for about 25 min. afterwards, i remembered i had to go to the post office wich was about an additional ten to fifteen minutes out of the way ( all together making it about 25 min ride to her house not including the 5 min at the post office getting stamps and mailing bills). i finally get to her house its about an hour later total since i texted her and she flips out on me like it was the end of the world. she still wasnt entirely ready either. im pretty understanding but i dont understand why if only for and hour shed be so mad. we didnt even have plans to do anything! i could see if we had plans, or if i was several hours late. i just dont get it.