You haven't told us the original source of this question, or precisely quoted it, so it's possible that something critical is missing from the statement of the problem. It's also quite possible that the solution is just wrong.
However, a likely explanation of your confusion is in the idea of normalized binary floating point numbers with an implicit leading one bit. This scheme is used most prominently in IEEE-754 floating point.
The idea is that although we will only store 9 bits of the mantissa in memory, we will always normalize the exponent so that the first bit of the mantissa is a 1. We will implicitly assume (without explicitly storing this bit) that the first bit of the mantissa is 1. This gives us an extra bit in the mantissa at no cost in storage. The 10 bit mantissas then range from
.1000000000 (stored in memory as 000000000)
.1111111111 (stored in memory as 111111111)
In this scheme, a number with the smallest possible exponent is considered denormalized (or subnormal) so that the implicit leading one bit is not assumed. This allows for mantissas all the way down to
.000000001 (note that there are only 9 bits here!)
This extends the range of the floating point numbers, but at the cost of precision at the low end. On many processors, arithmetic on denormalized numbers can be substantially slower than arithmetic on normalized IEEE floating point numbers. In some cases (this is common on GPU's) the processor will simply round all denormalized results down to 0 rather than doing the extra processing to properly handle them.