Strange "quota" issue

Here's the environment:

RDBMS: 10.1.0.4 (standard edition)
OS : RHEL AS 3 User "X" has "RESOURCE" priviledge and NOT given explicit "quota" to tablespaces "DATA" and "INDX".

The tablespaces has datafiles with 1024MB and has AUTOEXTEND upto 2048MB.

There are 5 datafiles for each tablespaces.

Problem: The datafiles got full upto 99% of 1024MB but never extended.

         Started getting ORA-01536 "quota exceeded for tablespace DATA & INDX"

Fix: Given explicit "quota unlimited" to the USER for the tablespaces

Puzzled:

1. "Resource" priviledge automatically gives "unlimited quota" to tablespaces.
2. Why did the datafiles never extended?
3. How did this work all this time without the "quota" problem?

Any insights will be greatly appreciated.....

TIA

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