From the Zbar FAQs
The ZBar decoder enables only EAN-13 by default
and
The UPC-A symbology is the subset of EAN-13 that starts with a leading 0... You can choose to receive the 12-digit results instead by explicitly enabling UPC-A.
It sounds like EAN 13 is enabled. To enable Code 128, put the following snippet after you disable Interleaved 2 of 5 (I25)
[scanner setSymbology: ZBAR_CODE128
config: ZBAR_CFG_ENABLE
to: 1];
If you want strict control over what is enabled and disabled, disable all symbologies and selectively enable the ones you want
// Disable all symbologies
[scanner setSymbology: 0
config: ZBAR_CFG_ENABLE
to: 0];
// Enable EAN 13
[scanner setSymbology: ZBAR_EAN13
config: ZBAR_CFG_ENABLE
to: 1];
// Enable UPC-A
[scanner setSymbology: ZBAR_UPCA
config: ZBAR_CFG_ENABLE
to: 1];
// Enable Code 128
[scanner setSymbology: ZBAR_CODE128
config: ZBAR_CFG_ENABLE
to: 1]