In times of bad markets, investors don't just worry about the big things, they sweat the small stuff. For proof, consider this question...

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In times of bad markets, investors don’t just worry about the big things, they sweat the small stuff. For proof, consider this question from my mailbag, asking about a small issue that, in its own way, affects everyone who invests in mutual funds.

This question is from Peter R. in Richmond, Va.:

Q: I got my monthly confirmation from my mutual fund the other day and had my calculator out.

Every month, I invest $200. It’s a no-load fund, so everything should go straight into the fund.

For my last deposit, the fund was trading at $81.73, and the statement said I bought 2.447 shares.

But multiplying 2.447 by $81.73, it equals $199.99331. That’s a difference of less than a penny, and it happened a few times this year.

If this happens to a few million shareholders it could be a lot of money.

Does the fund manager get to keep the difference?

A: When you add up everybody’s half-pennies, the numbers could be pretty big.

After all, if the same thing happened to $1 billion in new deposits, the discrepancy would equal a cool $5 million.

That said, those half cents don’t actually get added up; even when the fates conspire to give the fund an edge for a month or two, the additional cash stays inside the fund and doesn’t go to the manager.

Most fund companies break transactions out to two decimal places when it comes to calculating net asset value and three places when computing the number of shares involved in a transaction.

Behind the scenes, many fund firms work the numbers out as far as seven or eight decimal places.

They’d bring that complexity to the consumer if they thought the system was costing the firm a lot of dollars over time.

When it comes to making the cutoff, firms can truncate all numbers — simply stopping the calculation after the third or fourth place to the right of the decimal point — or can round up or down.

The result, as shown in Peter’s example, is that a transaction can come up a hair short.

This month, however, when the date of Peter’s automatic monthly deposit came up, the same fund was trading for $77.68 a share, and Peter got 2.575 shares for his $200.

The actual value of his transaction — shares multiplied by share price — was $200.026.

Over the two-month period, therefore, Peter actually wound up almost 2 cents ahead.

And some other shareholder, investing a slightly different amount, would have the numbers work out.

Over the course of months and years, this “slippage” or “breakage” evens out, staying insignificant and never growing into some huge windfall or shortfall for the fund.

Moreover, the money belongs to the fund, not to the manager.

As such, if it ever grew into an amount as big as a penny or two per outstanding share, the fund would likely make an adjustment and pay it out to investors.

Chuck Jaffe is a senior columnist at MarketWatch columnist. He can be reached at cjaffe@marketwatch.com or Box 70, Cohasset, MA 02025-0070.