I tried this too (as well as getinfo) - but it anytime gives the same difficulty value as getdifficulty.
13:07:03
{
"version" : "v1.5.0.0",
"protocolversion" : 60006,
"walletversion" : 60000,
"balance" : 56620.01000000,
"newmint" : 0.00000000,
"stake" : 0.00000000,
"blocks" : 377680,
"moneysupply" : 28043882.93443700,
"connections" : 5,
"proxy" : "",
"ip" : "178.5.80.82",
"difficulty" : 0.01380882,
"testnet" : false,
"keypoololdest" : 1387453236,
"keypoolsize" : 102,
"paytxfee" : 0.00000000,
"errors" : ""
}
13:07:06
{
"proof-of-work" : 0.01380882,
"proof-of-stake" : 0.00024414,
"search-interval" : 1
}
13:07:08
{
"blocks" : 377680,
"currentblocksize" : 1000,
"currentblocktx" : 0,
"difficulty" : 0.01380882,
"errors" : "",
"generate" : false,
"genproclimit" : -1,
"hashespersec" : 0,
"networkhashps" : 1872897,
"pooledtx" : 0,
"testnet" : false
}