Request performance don't know what's going on

I had problems with my request, which took 17 seconds to complete (lines 350 thousand):

SELECT idgps_unit, MAX(dt) 
         FROM gps_unit_location
        GROUP BY 1

Explain

1   SIMPLE  gps_unit_location   index       fk_gps2 5       422633  

After playing with this, I came up with this solution, which takes 1 second:

Select idgps_unit, MAX(dt) from (
SELECT idgps_unit,  dt
         FROM gps_unit_location
) d1
Group by 1

I explain:

1   PRIMARY <derived2>  ALL                 423344  Using temporary; Using filesort
2   DERIVED gps_unit_location   index       gps_unit_location_dt_gpsid  10      422617  Using index

And now I'm confused: why query # 2 is fast, and query # 1 seems to be the same query and seems to be written more efficiently.

Index1: DT, Index2: idgps_unit, Index3: idgps_unit + DT

Lead time agreed; request number 1 always takes 17-19 seconds; and # 1 <1sec.

I am using Godaddy VPS Windows Server 2008 Economy

Example table:

id | idgps_unit | dt | location
1 | 1 | 2012-01-01 | 1
2 | 1 | 2012-01-02 | 2
3 | 2 | 2012-01-03 | 3
4 | 2 | 2012-01-04 | 4
5 | 3 | 2012-01-05 | 5