Bits are not frequencies. If a position only contains an A (position 3 for example) then you would need 2 questions (bits) to derive that information without a priori knowledge.
- Is it a G or C? if no > then it is a A or T
- Is it T? If no then it is an A
Position 1 can be derived by 1 question only:
- is it a G or C? If no then it is an A or T
In this case the number of bits can be calculated using the frequenci)es.
For 4 bases, A only (100%):
4 * -0.25 * log2(0.25) - (-1 * log2(1)) = 2 (bits)
For 8 letters, A only (100%):
8 * -1/8 * log(1/8)- (-1 * log2(1)) = 3 (bits)
Mixed base possibility (A/T 50% each) with 4 bases:
4 * -0.25 * log2(0.25) - (-0.5 * log2(0.5)+-0.5 * log2(0.5)) = 1 (bit)
As mentioned in the comments, you can follow the wiki link for an example.
Or, an example with a little more explanation from the biology SE site.
sequence logo calcuation
Hopes this helps