A day before Apple (AAPL) is scheduled to report its 3rd quarter earnings, Robert Paul Leitao thinks he has put his finger on what’s wrong with the Street’s view of the company.

Leitao, who rides herd over 30 amateur analysts at The Mac Observer’s Apple Finance Board, has been tracking the gap between Apple’s revenue and its earnings. Both are growing like gangbusters, but earnings have been growing faster than revenue. The key, according to Leitao, can be found in what the accountants call OpEx.

In the chart above, from his Posts at Eventide blog, he traces the growing gap over the past six quarters between Apple’s revenue and its operating expenses (which include R&D and the broad category of general, selling and administrative expenses).

What this means is that more of Apple’s sales dollars are flowing directly to its bottom line — over the last six months, nearly $12 billion of net income was generated from $51.4 billion in sales, or an enviable 23.33%.

Looking at the data we collected from nearly 50 professional and amateur analysts for Fortune’s Q3 2011 earnings smackdown, Leitao concludes that the amateurs, by and large, have taken this declining OpEx percentage into account and the pros have not.

Below the fold, we have posted a portion of a spreadsheet Leitao created that sorts the analysts’ estimates based on how far they stray from that 23.33.%

“While some variances are expected in the estimates,” he writes, “the size of the variances from many of the pros are a bit startling and lead to significant underestimates of EPS [earnings per share].”

“At some point,” he concludes, “the pros need to update/revise their expense to revenue ratios to reflect the sharp drop in operating expenses relative to revenue, the attractive gross margins and lower tax rates that Apple has reported over the past few quarters especially. When that happens the pros will need to revise upward their eps estimates and corresponding target prices for the September quarter and FY2012.”

Below: A snapshot of Leitao’s net income variance spreadsheet. Note the amateurs (in blue) bunched at the bottom.

We’ll find out whose estimates were closest to the mark when Apple reports its Q3 earnings Tuesday at about 4:30 p.m. EST (1:30 p.m. PST). The earnings call with analysts that begins at 5 p.m. EST will be webcast here.