The weight per m is 0.3*0.3*2400 216kgf. The length they are using is 2*939.6/216 = 8.7m. They appear to be deducting 15cm from each column (or from each end of the beam) to account for the fact that your dimensions were centre to centre of member. Your calculation includes the overlap twice, so they are correct.
Edit: Oops, I nearly fell into the same trap. I think your figure is correct. The total length of two columns and one beam should be 9m, which makes your calculation correct.
BA