I agree with your overall point: it most likely does not matter when the values are close enough. However :) There can be two companies with 100M market cap. C

I agree with your overall point: it most likely does not matter when the values are close enough. However :) There can be two companies with 100M market cap. Corp A has issued 10M shares @ 10 each, Corp B has 10B shares priced at 0.01 A +/-0.001 change in Corp A share price is just 0.01% and moves the market cap by +/-10k, so probably nothing significant. The same nominal change in Corp B amounts to 10%, or +/- 10M in the company value, which is quite a big deal. Also I think there may be some money to be made in changes at the 7th decimal place with large enough volume of high frequency transactions.