They do that because the management and then the project management has decided it has to be done. ;-)
It depends very much on the requirements of the sales department and the controlling, as well as on the IT project management and the interfaces to their SAP system. Zillions of parameters.
If not "agile" project management, there is, after a 'development freeze' of a certain version a number of QA tests and stages, usually a rollout date which everybody has to be aware of. Bugs are fixed, testing goes back to the beginning of the current stage and if everything passed the QA checks the next step is carried out which follows the same rules. The 'rollout' is to a certain degree subject to the same procedures. 'to a certain degree' because going public can hardly be simulated (as we have seen with the first version of the configurator).
The change to an estimated delivery date at Ineos is likely closely dependent on the SAP software. And non-trivial changes in SAP is a process which can often last for months. Apparently, if I got this right, they are now close to the rollout of a new rendering for end users (otherwise the build slots wouldn't have disappeared). If something goes wrong, it might take a while to get it right because massive systems like SAP follow very strict rules of fixing, QA, documentation and all the meeting-discussion-protocols-etc fuss.
"Agile" development is very different, but I suspect that they are following the "v-model", which basically is what I described above.