If you're looking at the figures as being 'correct' or 'incorrect', that's really a misunderstanding of how to read these figures.
These figures exist with a context, in this case a planning document, so they are being presented to tell a specific story. Therefore, they should be 'correct' within that context. You are likely to find that context in how the planning documentation describes what the figures are demonstrating (e.g. 'the graph demonstrates that visitor numbers fluctuate and that, without investment in new rides, visitor numbers would decline', for example)
However, unless the document has been specific about how 'attendance' is being calculated, it will be very difficult to understand how these figure reflect reality outside this context - it is difficult to confidently reuse these types of figures unless you've been given the calculation behind the figure.
For example, are these figures discussing attendance to the theme park, or the whole resort? What is an 'attendance'? How does it account in attendance in the water park, golf and conference centre, or guests who might have only stayed at the hotels? Is there a chance that a single person might be duplicated if they used two different service offered by the resort in different orders? And has the way that's been calculated been consistent over the time period stated?
These variables might explain why the figures for Thorpe Park seem to be more consistent - there are a lot less variables in place at Thorpe, as you could basically use the entrances at the park turnstiles to work out attendance. The calculation is much more difficult if you're trying to show the attendance for the whole Alton Towers Resort.