That's quite normal. At least in the US and most other industrialized nations.
If you spend a lot of your gross profit you end up with a lot less taxes. That's why companies are always juggling their purchases to mitigate their tax payments. In real life there's countless different ways, even within the same country depending on which accounting rules a company wishes to follow, to pay or not pay taxes on purchases, what percentage of capital purchases are taxable in which year, and/or how depreciation of an asset is handled.
In the simplest most abstract form that we can simulate a wide variety of tax laws in the easiest way possible, the way it's handled now is just fine. Out of all the various ways the huge number of 'features' the game has are handled, I think the way it does taxes is probably the thing that needs the least amount of work, IMHO (that's not to say the game needs a lot of work, just taxes I'd spend the least amount of time on).
The only way to improve it significantly would be to model depreciation and good luck with that. There's about 10 thousand different ways to do it, none of which are particularly easy, and companies often change their accounting methodology throughout their life as their needs change (i.e. a regional airline that leased wouldn't handle their taxes the same as a legacy airline that owned)
Don't look at is as you made a "mistake" by buying your AVROs now instead of at the end of last year. Because all you did was change the year you're getting the tax credit. You're still going to get just as much money back. It's just instead of a lower tax bill for the month of December, you're going to have a lower tax bill for the month of January. If you had bought them in December, you'd just have paid more taxes in January. The end result is exactly the same.