Very few of the common methods of assessing body composition are so accurate. That is, they can be under ideal conditions—a highly skilled tester, excellent equipment, euhydrated state, and/or typical body electrolyte levels—but very rarely do these factors come together. So unless you are having your body composition assessed professionally, you should assume a high degree of uncertainty in the measurements.
However, if were are to assume, hypothetically, that the measurement is perfectly accurate, it does indeed mean that you will have lost 2.5 kilograms of body fat, in accordance with your calculation. But it would be highly unusual (read: entirely improbable, if not impossible) that muscle would account for the remaining mass.
Significant fat loss is always associated with some muscle loss, but never in such proportions. The bulk of the difference is almost always fluid loss, which is the consequence of muscle and liver glycogen depletion. Even under normal conditions, our mass can fluctuate by 1-2 kilograms. Under a restrictive diet and/or intensive exercise regime, the difference can be considerably greater.
Thus, what you are witnessing is most likely a combination of measurement uncertainty and glycogen-associated fluid loss.
I hope that helps.