It has to do with the difference in the gearing on first, second and third - the first on the auto is longer but the second and third are shorter to catch up plus the shift times as mentioned before help.

Automatic, Five-Speed Overdrive:

Torque Converter Stall Torque Ratio 1.800

Gear Ratios

1st 3.950:1
2nd 2.423:1
3rd 1.486:1
4th 1:1
5th 0.833:1
Reverse 3.147/1.93
Final Drive Ratio 3.270
Overall Top Gear 2.740

Manual, Six-Speed Overdrive

Description Synchronized in all forward gears

Gear Ratios

1st 4.459:1 --------- good to 30mph
2nd 2.614:1 -------- good to 50mph
3rd 1.723:1 --------- good to 80mph
4th 1.245:1 --------- good to 110 mph
5th 1:1
6th 0.838:1
Reverse 4.062
Final Drive Ratio 3.270
Overall Top Gear 2.740