@confusedHSA41
OK, I now have the right numbers. It turns out that because you didn't know the strange rule about coverage on the first of the month, that you probably overestimated your actual allowed coverage for 2018.
This is the way it works is this:
The two of you had Self-only coverage for 6 months and Family coverage for 6 months. So, using the formula for line 3 on page 4 of the 8889 Instructions, you have 6 months at $3,450 and 6 months at $6,900 (remember that this is for 2018). This totals to $67,500, which, when you divide by 12 months is $5,175 for the two of you to share.
If you think about it, your total contributions in 2018 were
Taxpayer contribution in 2018 - $3,450
Spouse contribution in 2018 - $2,465
Total contributions in 2018 - $5,915
So, clearly, no matter how you allocate the contribution limits for 2018, the two of you overcontributed by $740 ($5,915-$5,175).
The problem is that the TurboTax instructions told you to copy down what was on line Line 5 minus line 6 on your 8889s. But the 2018 tax return was done assuming that you could use the last month rule, so it didn't matter what TurboTax did on the 8889.
As a result, TurboTax arbitrarily allocated $3,450 of the $6,900 to each spouse. That's why line 5 minus line 6 on your 8889s had $3,450.
But the net effect of entering it this way caused some of the $5,175 available contribution limit to not be allocated to either spouse, hence the added income on line 18 of the two 8889s is too high.
Instead - and I agree that the instructions do not tell you this - you need to enter different numbers for line 5 minus line 6.
Line 5 minus line 6 is the amount of the contribution limit you are giving to your spouse. So your contribution limit is the $5,175 less the amount you allocated to your spouse.
If you would enter $1,725 on line 5 minus line 6 on your part of the interview, then you are left with $3,450 as your contribution limit. Since this is what you contributed, you have zero additional income added to your line 18 on your 8889.
However, in order for your spouse to show only $1,725 as her contribution limit, she has to assign $3,450 of the $51,75 to you - hence, her line 5 minus line 6 should be $3,450 (which is what you probably entered anyway).
This leaves her the contribution limit of $1,725. And since she contributed $2,465, she went over the limit by $740, which is what we saw above.
Now, it makes absolutely no difference who "overcontributed", because you are filing a joint return. Change the entries so that you have the $740 on line 18; it makes no difference.
But now, at least, you understand why you owe a penalty for "failure to maintain HDHP coverage", and why the TurboTax instructions misled you into owing more of a penalty than you actually should have.
I will raise this issue with the developers.
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