Why is the default allocation unit size smaller (2048 bytes) for exFAT than for NTFS (4096 bytes) when formatting a disk on Windows 7?

Question

I see that the default allocation unit size is smaller for exFAT (2048 bytes) than for NTFS (4096 bytes) when formatting a disk on Windows 7 via Windows Explorer:

Why is the default allocation unit size smaller (2048 bytes) for exFAT than for NTFS (4096 bytes)?