Calculating a known and deadly accurate interrupt frequency rate (pic multicalc etc)

[HankMcSpank]

Ok, so I'm back to PICs after an enforced time away...I'm wanting to get stuck into DDS, but straight out the blocks, I'm confused (it doesn't take much!)

Right, I want a steady and accurate 20khz interrupt rate - being of the lazy ilk, my first port of call was mister_e's mutlicalc ('Timer helper' tab V1.3.1 dated 2006 - is this the latest version?)

I'm using timer2 (8 bit timer)
Oscillator is running @ 16Mhz
The calculator said I should use a preloader of 63

So I knocked up this little program...

@ __CONFIG _CONFIG1, _FCMEN_OFF & _FOSC_INTOSC & _WDTE_OFF & _MCLRE_OFF & _CP_OFF & _IESO_OFF & _BOREN_OFF & _PWRTE_OFF & _LVP_OFF
@ __CONFIG _CONFIG2, _LVP_OFF
INCLUDE "DT_INTS-14.bas" ' Base Interrupt System
Osccon = %01111010   'sets the internal oscillator to 16Mhz
DEFINE  OSC 16
TrisA.5 = 0     'Pin2 an output.     
CM1CON0 = 0   ' COMPARATORS OFF
CM2CON0 = 0   ' COMPARATORS OFF
ASM
INT_LIST  macro ; IntSource,    Label,         Type, ResetFlag?
    INT_Handler  TMR2_INT,  _DDS,  asm,  YES
    endm
    INT_CREATE       ; Creates the interrupt processor
ENDASM
T2CON.2 = 1   ' Timer2 on
PR2 =  63     ' this should yield an interrupt rate of 20khz at 16Mhz.
 
@ INT_ENABLE TMR2_INT
Main:
    pause 100
    goto main   
END
'+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
DDS:
toggle PORTA.5
@ INT_RETURN

But when I actually scope pin 2 - I see a frequency of 31.2Khz - now bearing in mind because I'm toggling, the scoped frequency will be half of the actual interrupt frequency, therefore there must be an interrupt frequency of 62.4khz going on with the PR2 value?!!.

So what am not doing right?!! (it's sure to be embarrasingly simple!)

(oh, btw whats's the purpose of the 'reload (instr cycle)' slider in mister_e's calculator??)

So after dicking about for a while, I decided that better get my backside in gear & knock up a spreadsheet to work this all out...

Oscillator Freq         16000000Hz
Instruction clock        4000000Hz
Time per instru       0.00000025 Seconds
PR2 set to                   200 counts
Interrupt takes          0.00005 Seconds
Interrupt Frequency        20000Hz
Scoped  Toggle rate        10000Hz

Now when I scope pin2, I see something more like the expected result - 9.93khz ....but where did the other .07khz go?!!

So what am I missing to get an accurate interrupt toggle frequency reading of 10khz? (which would be a 20khz interrupt rate)