I bought some 1 amp fuses off of Ebay, and I hooked one up to see how accurate the rating was. To my surprise, it didn't blow anywhere near 1 amp. As a matter of fact, I used my PSU to step up the current 100mA at a time, and found that it blew at 1.8 amps!
Sounds like it's perfectly in typical spec. For accurate specs, you need to know the exact part number and look at the datasheet.
At 1.8*nominal current, expect a few seconds of blow time, but it may vary from half a second to minutes.
Had you left it running at, say 1.6A, it would have
eventually blown.
At 1.0A, it's not supposed to blow, ever (or the rating is in hundreds/thousands of hours; this varies as well). This is how 99.9% of fuses are rated; for specifics, you just need the actual datasheet.
Remember that fuses have quite some unit-to-unit variations. Really good datasheets show "thick" curve sets over the whole range; many datasheets just show the full curves for a "typical" unit, and maybe a few separate "guaranteed" numbers.
For example, at 2*Inominal, a fuse may be specified to blow between 1 seconds and 3 seconds.
Or, some fuse datasheet might outright forbid any continued use between 1*Inominal and 2*Inominal.
Fuses are inaccurate, but extremely robust (when chosen right with enough current and voltage breaking capability, remember that DC ratings are different from AC ratings). But for really accurate overcurrent protection, you need to use active smart semiconductor devices (and then you need an additional traditional fuse to protect against a fire in case this accurate device fails; but such a fuse only protects against fire when the electronics have died already, and as such, don't need to be user-replaceable.)