SessionSecurityToken.ValidTo and TokenCacheItem.Expires mismatch

Question

We're writing a custom SessionSecurityTokenCache so that our FedAuth cookies are valid across a webfarm with IsReferenceMode = true. We are also trying to enable sliding sessions, but it opens up a few questions around token expiration:

1. In Global.asax, we are handling the SessionAuthenticationModule_SessionSecurityTokenReceived event to check the remaining time on the token and reissue it if it is due to expire (within 5 mins, for example). In here, when we check token.ValidTo, it does not equal the expiryTime of the TokenCacheItem that we retrieved from cache. Why? How is TokenCacheItem.Expires supposed to be used?
2. In the AddOrUpdate method you're required to override, there's an expiryTime parameter; what is the intended use of this? Also asked here.