I think that the assumption is that a professionally serviced lifejacket WILL inflate whereas an amateur serviced one might not.
As a matter of interest, does your simple lifejacket have a crutch strap? The life jacket will always bob merrily on the surface of the waves, whether or not your face is up there next to it depends on how tightly your body is attached to it. The jacket will always go up, your body will always go down. Being suspended an inch below the surface of the water just gives your relatives the chance to bury your body.
To anyone contemplating it - buy a lifejacket with a light and a crutch strap.
At the risk of getting this post binned - I can recommend the Southern England lifejacket/dinghy suppliers in Basildon.