Many businesses that listed through special-purpose acquisition companies have failed to meet targets. Opendoor is a rare outlier. Back in September 2020, Opendoor was forecasting $9.8 billion in total revenue for 2023. As of the fourth quarter of last year, the company was already operating at a $15 billion annual run-rate for its core iBuying business alone.

Call it the

Zillow

Z 0.91%

fear factor. Before its decision to wind down its massive bet on iBuying, Zillow Group had been targeting a return on homes sold before interest expense of between plus or minus 200 basis points of break-even. The swings were much greater than that. In November, the company cited unpredictability in forecasting home prices as the reason why it was exiting the business.

Perhaps for that reason, investors were spooked when Opendoor said last week its fourth-quarter contribution margin was 4%, down from 6.5% a quarter earlier and 12.6% a year earlier. While there were certainly changing macroeconomic conditions involved, it is worth noting that its contribution margin declines came despite the fact that Opendoor generated $3.8 billion in revenue in the fourth quarter. That is more than 14 times what the company put up in the same period a year earlier.

Some analysts think Opendoor’s story is misunderstood. Wedbush’s Ygal Arounian expects margins to improve sequentially in the first quarter, noting Opendoor’s “very healthy” inventory quality. Indeed, Opendoor said as of year-end that just 8% of its homes had been in the market for more than 120 days—significantly below the equivalent percentage of comparable homes in the broader market.

But that might have something to do with the percentage of homes Opendoor, a company that continues to tout its goal to build a world-class consumer experience, has recently been selling to investors. Citing database-management company Attom Data Solutions and public property records, scholar-in-residence at the University of Colorado Boulder

Mike DelPrete

estimates Opendoor sold a whopping 22% of its inventory to investors last year. YipitData estimates that more than 10.5% of Opendoor’s inventory was sold directly to investors in the fourth quarter without ever being listed on Opendoor.com—up from 2.2% in the first quarter of 2019.

While quickly selling to investors likely lowers holding hosts for iBuyers, there are likely only so many homes Opendoor can sell this way, if it wants to be a consumer-first company. Furthermore, there could one day be legal limits: As of November, Los Angeles city lawmakers, for example, were looking for ways to thwart iBuyers’ ability to buy some single-family homes.

As far as consumer sales go, rising mortgage rates could deter future buyers from entering the market this year, potentially increasing holding times for iBuyers. Opendoor finances its own homes through debt, making it somewhat subject to the risk of rising interest rates. The company said last week it is expecting just a 20 to 30 basis point increase in unit costs because of rate increases over the course of 2022. But it had approximately $6.1 billion of non-recourse asset-backed loans as of Dec. 31, 2021, with a total borrowing capacity of non-recourse asset-backed debt of $10.8 billion, according to its fourth-quarter filing. It also notes that the company’s leverage could have meaningful consequences, including vulnerability in an economic turndown.

Opendoor more than doubled its market footprint last year and ended 2021 with an inventory balance of 17,009 homes across 44 markets representing $6.1 billion in value. A September 2020 investor presentation shows a goal of one day operating in 100 U.S. markets, generating a run-rate of $50 billion.

Give the company credit for aiming high: Back in 2019, Zillow said it expected to generate $20 billion in iBuying by 2024. We all know how that turned out.

Write to Laura Forman at laura.forman@wsj.com