There is a semantic difference: 1,000,000 == 1,000,000 where-as 1.00E+6 >= 1,000,000 < 1,010,000. The decimal places after the 1 in 1.00E+6 specify the precision of the measurement.
I don't think they specify any precision. It's just a way to write very large/small numbers approximately. (Though these numbers here aren't really considered large.)
