If I recall, some kinds of military training give their students a really broad grounding in everything, so that an Engineer would, for instance, know every Engineering speciality to Engineering-2, and a Jump specialist would just have Engineering (Jump)-3.
So characters with, say, Drive-2 could make any kind of ground vehicle go, but they could have a speciality of Drive (Tracked)-4, making them a specialist in driving tracked vehicles.
If you assume that every character received a generalist training, particularly in skills such as Science, they would have enough skill levels to handle any basic task requiring that skill with equal competence, and also be better at their speciality.
Assuming that Science skill is broad allows a physician character to use Medicine on aliens without penalty, because it will be assumed that the medic will have a solid grounding in specialities such as Biology, Xenobiology, Anatomy, Physiology and so on; and if they had Animals skill as well, that the Science skill would include sufficient grounding in veterinary science to permit the Traveller to perform veterinary medicine on an animal without worrying too much about whether or not they have Animals (Veterinarian), Science (Biology), Science (Pharmacology), Science (Anatomy), and so on.