This isn't necessarily unexpected. 32-bit platforms vs 64-bit platforms can make a significant difference, as well as the amount of data you're hashing.
$ uname -m
$ openssl speed sha256 sha512
The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes
sha256 29685.74k 79537.52k 148376.58k 186700.77k 196588.36k
sha512 23606.96k 96415.90k 173050.74k 253669.59k 291315.50
As you can see, on my 64-bit machine, SHA-512 beats SHA-256 for hashing anything more than 16 bytes of data at a time. And generally, the more data being hashed at once, the bigger the performance improvement.
Edit: As @MaartenBodewes points out in the comments, there's also SHA-512/256 which does the same computation as normal SHA-512 (with a different initial value) but truncates the output to 256 bits. This is a better option (due to the different IV) than simply truncating the output of SHA-512 to 256 bits by yourself in the case where you need the higher throughput but are limited to 256 bit outputs. Alternatively, if you really need higher throughput, BLAKE2b is an excellent cryptographic hash that is extremely fast and natively supports arbitrarily sized outputs (between 1 and 64 bytes).