It happens sometimes. It's my understanding that the 5888 is more pressurized than the 888. According to my source (a refill salesman), the metal body allows for pressure, making the refill write more smoothly and last longer.
I've had a couple "pop" the ceramic ball out of the refill creating a HUGE mess. I have MANY, MANY 888s in a box (I don't use them, just can't throw the away). Many of the 888s have been in the plastic storage box for 5 or 6 year, moved from "pillar to post" in all sorts of extreme climate conditions and yet the box is "clean".
Seems I "lose" a half dozen or so 5888s every year.
I DONT KNOW the real reason, but the "salesman's explication" seems as viable as any other explanation.