Sony split their phone models into groups based on hardware (like 3G, LTE, etc), and regions (like asian, europe,etc.). These are usually because the firmware has to be modified to work on each device (in case of hardware) or certain softwares which are very essential in certain regions maybe completely useless in other regions. So, each firmware varies from one another in these aspects. These firmware have different versions (like .757, .761, .769 as you said). All Xperia Z1 phones in the world which are active will be able to update to .757 firmware because it has Android Version Update which will be available for all models of a particular phone. While .761, .769 are bug fix updates which fix the bugs present in .757 . These bugs are usually occur when the base firmware is modified to work on other models of the phone. So, the models which has these bugs only will get this update, other models in which the Operating System runs without any problem will not get these updates.
Since you have not got any updates your phone should not have any bugs that are fixed in .761, .769 firmwares but sometimes the update just doesn't show up on the Update Center. So, better connect your phone to SUS to confirm that there is no new update for your phone.
Hope it helped.