Happens all the time.
Different types of banks make them up:
here's the $bank:
http://dynamic.nasdaq.com/dynamic/bank_1.stm
Whereas here's more than half of what makes up XLF:
Name Weight Shares Held
Jpmorgan Chase & Co 11.60% 18,639,068
Bank Of America Corporation 11.12% 47,000,640
Wells Fargo & Co New 9.73% 24,175,262
Goldman Sachs Group Inc 5.83% 2,431,507
Citigroup Inc 4.68% 92,230,190
American Express Co 3.26% 5,624,182
Us Bancorp Del 3.07% 9,045,048
Morgan Stanley 2.88% 6,429,420
Bank Of New York Mellon Cor 2.31% 5,695,353
Metlife Inc 2.08% 3,872,433
XLF is most of your major banks, whereas the $BANK is a broad spectrum including alot of smaller banks.
There's also SEF, which is short alot of the majors:
JPMorgan Chase & Co. 9.67%
Bank of America Corp. 8.21%
Wells Fargo & Co. 7.01%
Goldman Sachs Group Inc. 4.46%
Citigroup Inc. 4.12%
U.S. Bancorp 2.34%
Morgan Stanley 2.15%
American Express Co. 2.00%
Bank of New York Mellon Corp. 1.94%
VISA Inc. Cl A 1.78%
Since the Nasdaq tends to be a leader, many will use the $BANK index as an indicator for the broad markets. I like to keep both up on radar.