Dhrystone Benchmark

[Tower Ty]

I ran the above programs in D on my machine and got wildly varying results as soDhrystone Benchmark, Version 2.1 (Language: D)
Register option not selected.
Microseconds for one loop:     1.8
Dhrystones per second:   552669.4
VAX MIPS rating:    314.553

Dhrystone Benchmark, Version 2.1 (Language: D)
Register option not selected.
Microseconds for one loop:     0.3
Dhrystones per second:  3333333.3
VAX MIPS rating:   1897.173


It settled on 
Register option selected?  NO
Microseconds for one run through Dhrystone:     0.3
Dhrystones per Second:                       3333333.3
VAX MIPS rating =   1897.173


My question is why the massive variation  and why does the benchmark take the highest ?