Support Is there any consistency with 3g and 4g speeds?

I have had my evo 4g for about a month and Every other day I test out my 3g speeds at my house, work, School and random places. Well the 3g speeds that I get at my house are usually around 300kbps down and 400kbps up. But I just tested them today at my house in the exact same spot I usually do and I got 1,250 kbps down and 580 kbps up. Now I thought this was a glitch or something so I ran the test a few times again in a two hours and I keep averaging 1,300 kbps down and 580 kbps up. WHat would make my speeds faster and or slower ?