Indeed.
Let’s just rephrase your question:
Are all [random] Dallas DS18B20 [clones/counterfeits] always a degree low [than the original (specification)]?
Not necessarily but the chances are not low 
some of the counterfeit sensors actually do not work in parasitic power mode, have a high noise level, temperature offset outside the advertised ±0.5 °C band, do not contain an EEPROM, have bugs and unspecified failure rates, or differ in another unknown manner from the specifications in the Maxim datasheet.
Did you care to get your hold on a original one to compare them to your fakes/clones/counterfeits? Or did you just buy random? As a hint: From over 100 DS18B20 “type” sensor bought from aliexpress or ebay in the year 2019 and 2020 less than one (in numbers: 
) were originals 