How much can a scale vary? Am I crazy? :(

Emily
I have been EBF and concerned about my babies weight. He lost 10% at week one, and wasn't back at his birth weight until close to week 3.
I have a baby scale at home - I never really trusted this scale, until the past 5 days he seemed to have been gaining an oz a day, so I figured it's correct.
Last night I weighed him and it read: 8 lb 9.5 oz. he should be weighing more, but he's gaining at least..
Went to the pediatrician today, nurse says he weighs 9lb 14 oz!!!!!!!
I didn't see her weigh him, and I almost don't believe it.... 
Now that I'm home I'm nervous and wish I saw it for myself. I trust them.. I just don't understand how it could differ so much.
And he doesn't look 2+ lbs bigger?