Question

Why are some solar eclipses annular, but others total?

Answer 1

It's because the distance Earth-Moon varies, and so does the distance Earth-Sun.

Explanation:

The Earth moves around the Sun in an elliptic path, this means that the distance E-S varies, by about 3% a year.
The same goes for E-M (but in a lesser, and monthly, way).

Now if E-S is smaller, and E-M is larger, the Moon, as seen from here, can't just quite cover the solar disk, and we have an annular (=ring form) eclipse.

The other way around, we will have a total eclipse that will last a little bit longer than average.

Answer 2

When the observer O, Moon M and Sun S are aligned as S-M-O, total or annular solar eclipse occur. When the vertex V of Moon's umbra lies overhead. It is S-M-V-O alignment for annular eclipse. .

Explanation:

For total eclipse, the alignment will be S-M-O-V and V would be beneath O.
Note that MV is the axis of the Moon's umbra that revolves about the Sun.
When the umbra sweeps o, it is Total at O.

It is all a matter of distances, angles and proportions.