It's just an oddity of the way bonus spells are given in the 2nd edition. That is:
13 - 1st
14 - 1st
15 - 2nd
16 - 2nd
17 - 3rd
18 - 4th
19 - 1st and 4th
20 - 2nd and 4th
21 - 3rd and 5th
etc
I agree you could question whether 19 wisdom should give 1st and 3rd, rather than 1st and 4th - but then arguably that wouldn't have been a step up from the 4th level spell given at 18.
Many characters would have 21 wisdom on transfer to BG2EE after using the 3 tomes available in BGEE, which would reduce the disparity between 3rd and 4th level bonus spells.
Comments
13 - 1st
14 - 1st
15 - 2nd
16 - 2nd
17 - 3rd
18 - 4th
19 - 1st and 4th
20 - 2nd and 4th
21 - 3rd and 5th
etc
I agree you could question whether 19 wisdom should give 1st and 3rd, rather than 1st and 4th - but then arguably that wouldn't have been a step up from the 4th level spell given at 18.
Many characters would have 21 wisdom on transfer to BG2EE after using the 3 tomes available in BGEE, which would reduce the disparity between 3rd and 4th level bonus spells.