From the points made above (as an “agent”) I have been on the GDS at the same time and spoken to a colleague in Australia (I am in UK) and looking at the same flights we had different availability. Airlines yield management systems are getting cleverer and more sophisticated so they can tell where a TA is based and manage there availability and pricing accordingly. So sometimes it can make TA’s look stupid and expensive, others times the TA can come out better and we can be cheaper (along with the service element hopefully).
On the point of corporate travel, this is a classic point (especially I have found of US organisations) who book all travel in the “US” where booking locally could be much more effective cost wise and possibly from a User perspective as well. Not always the case – every scenario with flights seems to be different. For the sake of saving $1500+ you would hope the company travel policy is flexible enough to allow people to book “outside” the policy allowing for Duty of Care etc.