This is definitely an error. There are by definition 100 GBX in 1 GBP.
The cause is likely to be they are probably treating it as a cross rate via USD from experience. It is a weird error but it is understandable when changing all crossed currencies through the major. This is because in general the rate with respect to USD for any given currency is the most liquid market and as such when trying to determine the "cross rate" between two non-USD currencies those two rates are used, for example:
To get the GBPCHF cross rate the calculation is to cross the GBPUSD and USDCHF rates.
If the site is getting GBPUSD rates and (presumably) USDGBX or GBXUSD rates they will erroneously cross them expecting that to be the most liquid pair of contracts.
It is probably a good idea to raise this with that site.