Speaking as the genre tagger, I can explain the "low" number: it's not actually low! There are plenty of slash fics that aren't tagged slash. Unlike SWG and AO3 and other archives, they're not category tags; they're additional information tags people can add if they want. They're not mandatory.
The het/femslash/slash tags I only add if I'm the one deciding what tags go on the fanwork, mainly when the genre field is left blank. I only add those tags when the fanwork centers on the relationship. If the relationship is background-only, I mostly use gen under the principle that if background het is treated as gen, so should background slash.
If people write slash relationships but don't list slash in the header field and don't tag slash, then I don't add the slash tag. People decide what they want to put in the genre field in the header; what's there is literally all I will list in the tags. I don't want to take the risk of adding something that the poster may not want there or see as accurate.
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The het/femslash/slash tags I only add if I'm the one deciding what tags go on the fanwork, mainly when the genre field is left blank. I only add those tags when the fanwork centers on the relationship. If the relationship is background-only, I mostly use gen under the principle that if background het is treated as gen, so should background slash.
If people write slash relationships but don't list slash in the header field and don't tag slash, then I don't add the slash tag. People decide what they want to put in the genre field in the header; what's there is literally all I will list in the tags. I don't want to take the risk of adding something that the poster may not want there or see as accurate.