There must be something we don't understand.
I have built 143 lower tier seats, contracted on the 2nd, completed today the 7th. The estimate was 6 days (easy to simulate).
For the record and to make sure I am not mistaken :
Contraction :
http://img521.imageshack.us/i/arena1k.jpg/Completion :
http://img263.imageshack.us/i/arenan.jpg/It happens frequently that works at my arena are completed in advance and they were delayed by one day only a couple of times I think. So whether it's completely down to luck and I have been fortunate until now (it's possible, I have only had 14 expansions in total, a sample that is certainly much smaller than yours) or I do something by complete chance and it works well.
I am clueless how expansions work but I don't intend to break this golden cycle, I won't change anything. I usually build for 100k of one type of seats.
Happy New Year :)