Can anyone expain why we are getting the following warning in the
db2diag.log:

FUNCTION: DB2 UDB, Self tuning memory manager,
stmmDecreaseDBMemOriginatedBySTMM, probe:1606
MESSAGE : ZRC=0x02AE00A5=44957861=STMM_DBMEM_COULD_NOT_BE_DE CREASED
"The DATABASE_MEMORY parameter could not be decreased"
DATA #1 : String, 141 bytes
Overflow memory decrease amount exceeds overflow size.
DATABASE_MEMORY decrease failed. Decrease amount: 278200320, overflow
size: 226295808

Thank you.

Most likely its because memory is in use by DB2 (bufferpools and such)
after seeing this message in log db2 usualy shrinks bufferpools few
minutes later and then decreases DATABASE_MEMORY