Short roots can also be due to age or chemical damage. This damage is difficult to detect in the TZ test. If there is no damage visible in the seedling that produces short roots, damage is difficult to detect in the TZ test. Seedlings with short roots could be considered abnormal in a germination test but viable in a TZ test
 

Lettuce seedlings - variable root length
NORMAL and ABNORMAL

Lettuce embryo - TZ VIABLE
Different rates of emergence