MATCTX problem

I'm getting dubious results with MATCTX after upgrading to V5R2:

DCL DD     RECEIVER    AUTO BDRY(16) CHAR(256);
DCL DD     PROVIDED    DEF(RECEIVER) POS(1) BIN(4);
DCL DD     AVAILABLE   DEF(RECEIVER) POS(5) BIN(4);
DCL SPCPTR ?RECEIVER   AUTO INIT(RECEIVER);
DCL DD     X           AUTO BIN(4);
DCL DD     Y           AUTO BIN(4);

CPYNV   PROVIDED, 96;
MATCTX  ?RECEIVER, *, X'0500000000';
SUBN    X, AVAILABLE, 96;
DIV(S)  X, 32;          /* X = number of objects in machine context */

CPYNV   PROVIDED, 97;
MATCTX  ?RECEIVER, *, X'0500000000';
SUBN    Y, AVAILABLE, 96;
DIV(S)  Y, 32;          /* Y = number of objects in machine context */

BRK     "LOOK";
PEND;

I would expect either of these MATCTX's to calculate the same number of
objects in the machine context.

However, the debugger shows X=1203, and Y=722.

Why are X and Y unequal, depending if PROVIDED is 96 or not?

(By the way on V5R1 they are equal; on V5R2 they are unequal.)