finding the rate expression using colorimeter

i have done an experiment using colourimeter to find the rate expression of the reaction between iodine and propanone.

there were 3 sets of reaction mixtures

1. 4 cm3 of propanone, 2 cm3 of 0.05M iodine, 4 cm3 of 1M HCl, 4 cm3 of water

2. 2 cm3 of propanone, 2 cm3 of 0.05M iodine, 4 cm3 of 1M HCl, 6 cm3 of water

3. 4 cm3 of propanone, 2 cm3 of 0.05M iodine, 2 cm3 of 1M HCl, 6 cm3 of water

i obtained 3 graphs of [i2] against time, and i found the slopes of them

1. -0.00647

2. -0.00367

3. -0.00375

what are the possible explanations for the slope of set 3 slightly greater than that of set 2?! is it only becoz there's some error when plotting the graph, or there are other reasons behind?!

and also, i found that the initial temp of the 3 reaction mixtures being slightly different, why is it so? will this affect my result a litte? since temp affects the reaction rate..

thx alot!