976759808K == 976759808 * 1024 == 1000202043392 == 1TB
So it did what you expected. But that is a small matter.
I see your real problem though.. If you have 4 2TB devices and 2 1TB devices, md will only acknowledge 1TB of each device and you cannot grow to use more of fewer devices.
If you had 1 more 2TB device that you could swap for a 1TB device, then you could degraded the array so that it all lives on 2TB devices, then you could reshape. But I wouldn't recommend that - you should never discard your redundancy by choice - the risk is too high.
If you still have the 4 1TB devices that you replace, then you can use RAID0 to combine them into 2TB devices, and so make the whole array consist of 2TB devices (4 real, 2 RAID0). Then you should be able to reshape nicely.
It would be good to make md/mdadm handle this better - it isn't entirely straight-forward though. I'll have to give it some thought.