i sort of think "possibly" your engine was played with before you got it. that said, who really knows??? not sure about america, but i do know the 4k-e's in new zealand had dished pistons. maybe someone at some stage did a rebuild and couldn't source the original type pistons and had to go to flat tops, with a head change???
i know in the ACL catalog there are no aftermarket dished pistons available, but there is a part number for the thinner rings they run. incidently the 5k, and 4k dished pistons run 1.5mm compression rings. all flat top pistons use 2mm compression rings. the dished piston 4k cylinder heads i have are identical to the 5k castings which also use dished pistons.
i have had 3 dished 4k's now. they were all jap imports. the first i bought from an importers, it had a 4k-e sticker on the timing cover, but no manifolds. the piston on the right in the pic above came from it. i put 230,000 kays on that motor.
the second i got from Bec's brother in law, who had bought an import motor for his ke20. it blew a head gasket and i acquired the car. it's piston is the D shaped one on the left in the pic. the casting number on the inside of the piston identified it as a 4k-u piston.
the third engine i got from a guy i helped tidy up his yard.... the engine was in a ke30 which was a fresh import that they never finished hooking up. it was like new inside, and had the same pistons as the ones on the right in the pic. i gave that engine a cam and a few other tweaks and drove it for the three and a half years before my current 5k.