There aren't a range of poster templates that staff can have easy access to.
If a sign needs making the need often arises quite quickly. You make it, and you get it into use so that you can go back to doing what you need to do.
At the end of the day, very few front line staff know (or care) about the "design standards". They're there to get a job done, and they'll do that with whatever tools they have to hand. For more permanent signs ones to the 'official' standards will often be made up, but procurement of these can take some time.
Sadly the people who decree the signage standards at stations generally have very little front line experience. Commentators here probably have just as little experience - as long as the sign does the job, then it hardly matters what it looks like (signed someone who had to make up a *lot* of signs, including an entire set of directional posters for a weekend shutdown, at zero notice).
