Factor of a Mersenne number

It may be illustrative to write the numbers out in binary. I'll use $2^{21} - 1 = (2^7)^3 - 1$ instead of $2^{91} - 1$, since it's shorter:

$$\begin{aligned} 2^{21} - 1 &= \underbrace{111111111111111111111}_{21\text{ digits}}\,\vphantom1_2 \\ &= \underbrace{1111111}_{7\text{ digits}}\,\underbrace{1111111}_{7\text{ digits}}\,\underbrace{1111111}_{7\text{ digits}}\,\vphantom1_2 \\ &= 1111111_2 \times 100000010000001_2 \\ &= (2^7 - 1) \times (2^{14} + 2^7 + 1). \end{aligned}$$

We have $$2^{91}-1 = (2^7)^{13}-1 = (2^7-1)((2^7)^{12}+...+2^7+1)$$

More generally:

$$2^{ab}-1 = (2^a)^{b}-1 = (2^a-1)((2^a)^{b-1}+...+2^a+1)$$

so $2^a-1\mid 2^{ab}-1$

And we can replace $2$ with any other number.