What the heck is A+ ball and why has the top Yankee prospect, Gleyber Torres, played no higher?

Gleyber Torres
Position: Shortstop
Bats: Right, Throws: Right
Height: 6' 1", Weight: 175 lb.
Born: December 13, 1996 in Caracas, Venezuela (Age 20 and 41 days)

Prospect Ratings by Baseball America:
Pre-2016: Rated #41 Prospect

Prospect Ratings by MLB.com:
Pre-2016: Rated #28 Prospect

Prospect Ratings by BaseballProspectus.com:
Pre-2016: Rated #41 Prospect

Year	Age	AgeDif	Tm	Lg	Lev	Aff	G	PA	AB	R	H	2B	3B	HR	RBI	SB	CS	BB	SO	BA	OBP	SLG	OPS	TB	GDP	HBP	SH	SF	IBB
2014	17	-2.8	2 Teams	2 Lgs	Rk-A-	CHC	50	215	182	37	54	8	6	2	33	10	7	29	40	.297	.386	.440	.826	80	3	0	0	4	0
2014	17	-2.6	Cubs	ARIZ	Rk	CHC	43	183	154	33	43	6	3	1	29	8	7	25	33	.279	.372	.377	.748	58	1	0	0	4	0
2014	17	-4.2	Boise	NORW	A-	CHC	7	32	28	4	11	2	3	1	4	2	0	4	7	.393	.469	.786	1.254	22	2	0	0	0	0
2015	18	-3.5	2 Teams	2 Lgs	A-A+	CHC	126	538	487	54	140	24	5	3	64	22	14	44	115	.287	.346	.376	.722	183	7	2	1	4	1
2015	18	-3.4	South Bend	MIDW	A	CHC	119	514	464	53	136	24	5	3	62	22	13	43	108	.293	.353	.386	.739	179	7	2	1	4	1
2015	18	-4.7	Myrtle Beach	CARL	A+	CHC	7	24	23	1	4	0	0	0	2	0	1	1	7	.174	.208	.174	.382	4	0	0	0	0	0
2016	19	-3.6	2 Teams	2 Lgs	A+	CHC-NYY	125	547	478	81	129	29	5	11	66	21	13	58	110	.270	.354	.421	.775	201	8	5	5	1	0
2016	19	-3.5	Myrtle Beach	CARL	A+	CHC	94	409	356	62	98	23	3	9	47	19	10	42	87	.275	.359	.433	.791	154	5	5	5	1	0
2016	19	-3.7	Tampa	FLOR	A+	NYY	31	138	122	19	31	6	2	2	19	2	3	16	23	.254	.341	.385	.726	47	3	0	0	0	0
All Levels (3 Seasons)							301	1300	1147	172	323	61	16	16	163	53	34	131	265	.282	.356	.405	.761	464	18	7	6	9	1
A+ (2 seasons)					Minors		132	571	501	82	133	29	5	11	68	21	14	59	117	.265	.348	.409	.757	205	8	5	5	1	0

.775 OPS in 2016 in A+ ball. In 2017 Torres will undoubtedly be promoted to AA ball. Then eventually to AAA, then the New York Yankees.

But his batting numbers so far are nothing special. So, why is he rated so high when he's in the functional equivalent of ...? What?

https://en.wikipedia.org/wiki/Minor_League_Baseball#History

The current minor league classification system divides leagues into one of five classes, those being Triple-A (AAA), Double-A (AA), Class A (Single-A or A), Class A Short Season, and Rookie. Furthermore, Class A is further subdivided into Class A and Class A-Advanced (often called Low-A and High-A, respectively), and Rookie is further subdivided into Rookie Advanced, Complex-based Rookie and international summer baseball. Under the rules governing the affiliated minor leagues (specifically Major League Baseball Rule 51), Class A Short Season is a separate classification from the other leagues bearing the "Class A" name, despite the similarity in name.
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So is there: A+, A, A-?

Whatever level of ball Gleyber Torres has played so far, it's below AA. So, why are people so sure that he is the top Yankee prospect? Why not the other prize acquisition from July 2016, outfielder Clint Frazier, who has played AA and FINALLY after three pro years AAA, where Frazier has only 129 PA, .657 OPS?

Torres OPS exceeds that of Frazier but it's A+ v. AAA. And Frazier PA and OPS:
2015 A+ 588  .842
2016 AA 391  .825

Torres is a shortstop and Frazier an outfielder but Frazier has shown more with his bat, at least below AAA. So, what the heck?

2016 rankings (lower is better; 1 is best):
Baseball America: Frazier 44, Torres 41
MLB.com: Frazier 27, Torres 28
BaseballProspectus.com: Frazier 53, Torres 41