Sweater-grading math: fun? Or crazy-making?

How about some numbers? So I looked up the standard measurements for German baby clothing sizes and multiplied those chest measurements by my gauge (2.8sts/cm, if you must know) and got... some odd results. Frex, size 80 (10-12 months) and size 86 (13-18 months) came out pretty much the same, so I have all these little two-month ranges and then suddenly this sweater will fit your infant for nearly a year. That seems unlikely. Quite a few of these sizes seem profoundly unlikely, actually.

Size	50	56	62	68	74	80-86	92	98	104-110	116	122
Fits	newborn	1-2m	2-3m	4-6m	7-9m	10-18m	1.5-2y	2.5-3y	3.5-5y	5.5-6y	6-6.5y
cast on	80	88	88	96	96	104	104	112	112	120	120
sts/section	10	11	11	12	12	13	13	14	14	15	15
Increase rounds
r4 r12 r20 r28 r36 r44	14 18 22 25 — —	15 20 24 28 — —	15 20 24 29 — —	16 20 24 28 30 —	16 20 24 28 32 —	18 22 26 30 33 —	18 22 26 30 34 —	18 22 26 30 33 35	18 22 26 30 33 36	20 24 28 32 35 37	20 24 28 32 35 39
Divide after r	32	36	36	38	42	42	42	44	44	48	48
Stitch count
Total per section	200 25	224 28	232 29	240 30	256 32	264 33	272 34	280 35	288 36	296 37	312 39
Front & back	55	62	64	66	72	74	76	75	80	82	87
sleeves	45	50	52	54	56	58	60	62	64	66	70

[I don't know why Blogger's inserting these weird indents.]

Bear in mind that all of these numbers were achieved with an online baby size chart and the calculator in my phone. No knitting was perpetrated on any of these sizes except for the smallest one. 

So yeah, I'll probably need some test knitters.