Acia Saplings turning into Great oaks

When I put down Acia saplings in 2 by 2 block, thinking I might get some other new tree like the great oak but instead I actually got a great oak after putting down the bonemeal

Steps to Reproduce:
1. Put acia saplings in a 2 by 2 square a = sapling
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2. put down some bonemeal
3. congrats! you have a great oak!