Perhaps because the built-in (continuous) test system will generate flags and remove data on your instruments.
Having experienced a failed ADIRU due water ingestion I can safely say this is not necessarily the case. The instruments will show various errors but may not be removed from view until the unit output fails for the instrument in question.
Initial indications included IAS DISAGREE, AOA DISAGREE, ALT DISAGREE, A/T LIM, then --- on upper DU, lost predictive information in FMC...
These failures developed into no altimeter tape, no speed tape, on failed side.
All failed indications subsequently recovered 30-45 minutes later into the flight, engineers explained this as the water had evaporated by then as they discovered the signs of water ingestion on the ADIRU affected...
The water affecting the ADIRU was dripping from the Airstairs Drip Tray which had some cracked seals allowing it to sip into the E&E bay below.