why data length in CS101_ASDU_addPayload is different with data length in MessageQueue_enqueueASDU

[sdoa5335717]

In function CS101_ASDU_addPayload,
if (self->payloadSize + self->asduHeaderLength + size <= 256)...
the max length 256 is ASDU;
In function MessageQueue_enqueueASDU
int asduSize = asdu->asduHeaderLength + asdu->payloadSize;
if (asduSize > 256 - IEC60870_5_104_APCI_LENGTH) ...
the max length is more little value 250, that IEC60870_5_104_APCI_LENGTH is 6.

for example
I have data which length is 247, when I call CS101_ASDU_addPayload is success because 247+6<256, but when program call MessageQueue_enqueueASDU, asduSize is 247 + 6(asdu->asduHeaderLength) = 253, is bigger than 250(256 - IEC60870_5_104_APCI_LENGTH), so failed.

so I think (self->payloadSize + self->asduHeaderLength + size <= 250) is correct.