Lots of possible explanations, I am afraid:
- meters just aren't that accurate. 6.1 and 5.6 are close enough that the meter give that result if the readings were both at 5.9
- speed at which you ate will affect digestion time
- hunger level before food could have made your bg drop between testing at 6.1 and the blood glucose hitting your bloodstream
- the fat in the meal may have delayed digestion
- or the carbs in the meal may have peaked and be dropping by the time you tested the 5.6
(basically, you don't know if your 5.6 was a rising or a falling figure, and only the speed of your digestion and the carb/fat/protein ratios in the meal will determine that. Fibre is also a factor, but probably not with that particular meal)