Re: Cosmos from seed.
Yes, compost varies considerably doesn't it, Tess.
The soil based ones like John Innes 1, 2, 3, are soil like, as are the Multi-purpose 'with added John Innes.'
Then the ones without the JI are a different texture altogether, often with big dry lumps in it and fibrous, as you say.
I have had Ericaceous varying considerably too.