It sounds crazy, but yes, there are multiple classes within the standard first, business, and economy.
Each airline has their own method for dividing this up, but with Singapore they have:
Suites/First: RFAP O
Business: CDJZ I
Economy: SYBMHWUEKGQNVT X
So they've divided it up into a lot of classes!
What's the difference? Well from the same link:
For ex-SIN market, the order should be:
V = special promo fares,
G = Group fares,
N = GV4 fares,
Q = GV2 fares,
E = FIT 1 mth fare/ 14 days max for S.E.A. countries,
K = Interline Fares with other airlines, e.g. AA, TN, PS
M = RTW Fare
Y = 12 mths fare, can do open-jaw,
S = same as Y fare/ one-way fare
J = the restrictive business fare, 12 mths
C,Z = flexi business fare/one-way fare, 12 mths
F = no min/max stay
R = Suites class on a380 only
X, I, O = award redemption on the respective classes.
Sounds bizarre, after all, it's not location, it's more for pricing. Some of them are non-refundable tickets, some are cheaper, V is reserved for special promotions with LOTS of restrictions.
On your ticket there's usually an indication of what class of economy your ticket was, so check that and then match the terms and conditions of that with what they did. If your ticket says charges may apply for changes, then yeah, that's expected. If there are no restrictions on your ticket, then they were possibly 'cheating' you.