N is the cheapest revenue fare bucket.
The reason you didn't get to book into S or N is almost certainly not because the inventory ran out but because the fare rules or dates you were searching didn't have anything that works for the S or N buckets. Looking at the published fares in that market there is not even an N fare available so there's no way it would price there:
Code:
Flexible Fare Search -- EWR-NAS roundtrp for date range Aug-Feb on carrier CO -- Continental Airlines
Carrier|From |To |FareBasis |Fare |Min |Max |Adv. Purch|Earliest |Latest |
CO |EWR |NAS |SDA10SNN |USD$ 250 | | |10days | |15nov11 |
CO |EWR |NAS |WA10SNN |USD$ 281 | | |10days | |15nov11 |
CO |EWR |NAS |SA7BN |USD$ 351 | | |7days | | |
CO |EWR |NAS |WA7BN |USD$ 391 | | |7days | | |
CO |EWR |NAS |VA3BN |USD$ 453 | | |3days | | |
CO |EWR |NAS |WHOLN1 |USD$ 482 | | |seerules | | |
CO |EWR |NAS |QA3BN |USD$ 502 | | |3days | | |
CO |EWR |NAS |VHOLN1 |USD$ 542 | | |seerules | | |
CO |EWR |NAS |HA0BN |USD$ 562 | | |seerules | | |
CO |EWR |NAS |QHOLN1 |USD$ 593 | | |seerules | | |
CO |EWR |NAS |UA0BN |USD$ 643 | | |seerules | | |
CO |EWR |NAS |HHOLN1 |USD$ 653 | | |seerules | | |
CO |EWR |NAS |UHOLN1 |USD$ 734 | | |seerules | | |
CO |EWR |NAS |EA0BN |USD$ 825 | | |seerules | | |
CO |EWR |NAS |ZR14N |USD$ 895 | | |14days | | |
CO |EWR |NAS |VHOLN1 |USD$ 916 | | |seerules | | |
CO |EWR |NAS |QA0BN |USD$ 945 | | |seerules | | |
CO |EWR |NAS |EHOLN1 |USD$ 1017 | | |seerules | | |
CO |EWR |NAS |QHOLN1 |USD$ 1034 | | |seerules | | |
CO |EWR |NAS |ZRHOLNRO |USD$ 1097 |seerul| |seerules | | |
CO |EWR |NAS |HHOLNRO |USD$ 1116 | | |seerules | | |
CO |EWR |NAS |MHOLN1 |USD$ 1135 | | |seerules | | |
CO |EWR |NAS |MHOLNRO |USD$ 1218 | | |seerules | | |
CO |EWR |NAS |ZOX |USD$ 1261 | | |seerules | | |
CO |EWR |NAS |YOX |USD$ 1298 | | |seerules | | |
CO |EWR |NAS |ZHOLNRO |USD$ 1462 | | |seerules | | |
CO |EWR |NAS |YHOLNRO |USD$ 1501 | | |seerules | | |
CO |EWR |NAS |COX |USD$ 1708 | | |seerules | | |
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Reading the fare rules for that cheapest S fare you'll eventually find this nugget:
Fare Rules Export from Wandering Aramean Travel Tools
Code:
Fare details for SDA10SNN
INSTANT PURCHASE
...
DAY/TIME
PERMITTED TUE/WED FROM THE GATEWAY.
....
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Since you are traveling on a Sunday that's not an option for you. Hence the W fare.