Usually because the data is published exactly as AMFI filed it, and the oddity is real rather than a bug. Silently correcting someone else's filing would make the data disagree with the official record for reasons nobody could audit. Five that come up repeatedly:
1. An IDCW scheme shows roughly 0% return. This is correct and it is the one most likely to mislead you. NAV-based returns understate IDCW plans, because the payouts leave the NAV. One arbitrage fund's IDCW plan shows 0.00% over three years on NAV while its Growth twin shows 7.61%. Same fund, same portfolio. For any like-for-like comparison, use the Growth option.
2. The same fund appears under two different scheme codes. When a fund changes manager, AMFI issues a new scheme code while the ISIN stays with the instrument - so one code gives you only one leg of the history. Hundreds of ISINs sit on more than one code. To assemble the whole thing: search by ISIN with active=all (essential - without it the matured leg is hidden and you get half the story with no warning), pull each scheme code, then compare the NAVs either side of the boundary before joining them. That last step is not optional: usually the NAV runs continuously across the handover, but where units were exchanged at a ratio it jumps, and splicing those blind invents a return nobody earned. Whose track record survives a merger is a regulatory question, so the raw NAV is published as filed and the judgement is left to you.
3. A Gold ETF filed under an equity category. That is how AMFI filed it. Categories are published as received.
4. A matured scheme shows a NAV of 0.00 on its last day. That is AMFI's terminal marker, published as filed. Filter on the active flag if you only want live schemes.
5. Odd numbers in the older history. The archive carries a small number of source-filed oddities - a few bad prints, some par-value resets, occasional precision artefacts. Where the pull is faithful to what AMFI published, they are documented rather than silently corrected.
The practical rule: treat an oddity as a filing to understand, not a value to clean - and check whether it is one of these before you patch it in your own pipeline. If the number that looks wrong is an AUM, that has its own explanation; if a category count moved, that is SEBI's recategorisation. Source: https://www.tigzig.com/post/mf-nav-api-faq-aug2026.
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