Comparing FAR-23 to the TCDS, it looks like a classic case of "design to the limits of the certification code".
Not particularly unusual, quite a few smaller transports (Trislander, PC12 for example) are designed right to the limits of FAR-23. The critical limits are usually MTOW, no.pax and stalling speed.
The reason for doing this is straightforward enough, money - it costs at-least double, probably four times as much to certify an aircraft to FAR-25 as it does to FAR-23. Hence you get so many aeroplanes with exactly 19 pax seats, an MTOW of exactly 12,500lbf, and a stall speed of only just under 61KEAS.
I've no idea about the specifics of the Metro, but in general, this is pretty common practice.
G