In very simplistic terms, remember that only about 25% of the air going through your engine is used for combustion, the remainder is used for cooling. As you advance your power lever, particularly at lower Ng's(N1's), your increase of fuel to the fire is momentarily ahead of the increase of airflow
604 has explained it as simple as it can be made.
When you start a turbine you are always looking for a "hot start" because you are only turning the turbine with the starter to supply air and the fuel control is in the process of ramping up the fuel flow for idle with the air flow catching up as N1 comes up to speed.
norwesters question is just the same only higher up the rpm range.
VFD