You are misunderstang what happened. On July 3rd at 3:00 PM the Union forces on Cemetary Ridge numbered about 10,000 plus arty( a lot of it) and a little cavalry. The CSA attackers outnumbered the defenders 3:2 ratio. The Union line would have broke if the CSA cavalry had been there to support the flanks of the attack.
Picketts Division (brigades of Armistead, Kemper & Garnett) 5,040
Pettigrews Division (brigades of Fry, Marshall, Davis & Brockenbrough) 4,570.
Trimbles Division (brigades of Lowrance and Lane) 2,200
Total infantry attacking 11,800, plus 19 guns that moved forward with the assault. (Note that this is almost 1/4 less than what is traditionally discussed when discussing the "charge" of 15,000 men).
Front line in the center in Cemetery Ridge: Hays Division (brigades of Sherrill, Smyth, and the 8th Ohio under Sawyer) 2,000.
Gibbons Division (brigades of Webb, Hall & Harrow) 2,425.
Doubledays Division (brigades of Stannard & Gates (only 2 regiments each)) 1,720.
Humphreys Division (1st Mass Sharpshooters) 200.
Total infantry in front line 6,345, supported by 18 guns.
The Union reserve nearby totaled about 3,500, but only 2,560 (13 regiments and the 10th NY Provisional Guard) were actually moved up in support of the front line.
So, initially, 11,800 Confederates went against 6,345 Federals, but during the assault the Federals were strengthened while the Confederates declined.
My contact thinks that probably about half of the reserve actually got into combat. Nonetheless, at the critical point, the Federals probably possessed a slight numerical advantage as Brockenbroughs brigade of 700 failed to cross half the field and Confederate casualties were approaching 4,000 by the time the front brigades neared the angle. And that doesnt count the unwounded shirkers who melted away while crossing the fields.count
One should not include Wilcox and Langs Confederate brigades from Andersons Division totaling 1,500 men in the assault, as they moved forward late and after the main assault had already been stopped. They retreated after reaching Plum Run, were unable to keep Stannard from flanking Pickett on his right, and retreated as soon as they saw the assault had been repulsed.
In other words, perhaps---not counting reserves---the initial assault numbers would have been 3:2 or even 2:1, but by the time the Confederates got within shooting distance, their numbers had dropped to 7,000 vs. the original 6300 now reinforced by at least 2500, or 8800 to 7000---not even a 1:1 ratio.
I had forgotten that the Confed cavalry---already worn out, as I explained---had in fact been beaten earlier in a battle with the Federal cavalry and couldn't have helped if they wanted to.