Possible reasons for a difference between YTD P&L (Main TradeLog Grid Screen) and G&L on Form 4797 in MTM accounts after End Tax Year:
The main TradeLog grid shows the year-to-date (YTD) P&L for all trades currently displayed while the Form 4797 only shows the P&L for the current tax year.
If the file is set to "MTM First Year" and positions were open at the end of the prior non-MTM year, the YTD P&L will include the P&L from the prior year's open positions that had been marked to market on 12/31 of the prior year and re-opened on 01/01 of the current tax year. However, Form 4797 only reports P&L from the current tax year. (For this reason, first-year MTM filers must file the Section 481 Adjustment Report in addition to Form 4797 - see: Additional MTM Reports)
The main TradeLog grid may also include other instruments not reported on Form 4797 such as futures (unless the IRS Section 475(f) MTM Election was made on futures - please check with your tax advisor for more information) and currencies.
Matched open lots: If you manually matched open lots with the Edit>Match Tax Lots function (see: Match Tax Lots) before you ran the End Tax Year function, the matched open lots that were marked to market at year end do not include the closing MTM position on Form 4797 since the "M" record on 12/31 was not included in the matched lot.
Ideally, one should only match closed lots, never open lots in an MTM account. However, there is a workaround to fix this:
Run File > Unlock/Reverse End Tax Year, click Yes to "Keep MTM year end prices?"
From the "All Accounts" tab, click Find > TrNum, enter the lot/ trade number of the first trade that was manually matched and closed at MTM price
If you have more than one account, double-click on one of the records to pull it up in its individual account tab so you can edit
Select the "M" record on 12/31 of the current tax year
Click Edit > Edit Record
Enter the same "Matched" number in the "Matched" column as the other records in the same lot (see: Display Matched Tax Lots)
Hit your Enter key TWICE to save.
Do the same for any other lots/ trade numbers affected
When you are finished, you must re-run File > End Tax Year and allow TradeLog to overwrite the already existing file for the next year.
Re-run Form 4797 to check the new Gain/Loss result.