Because he is coming off his rookie contract and not his 2nd contract, he is only eligible to make 30% on his next contract, not 35%. Typically guys coming off of their rookie contracts can expect to make 25%, but there are incentives to push that number up to 30% (like an All-NBA Team selection).
Siakam, for example, will be getting the 28% max because he made 2nd Team All-NBA, but could have been at 29% had he made 1st Team. With Tatum, perhaps we could get him to lock-in that 5th year (no player option) with a full 30% guarantee, but I think we can expect something similar to Siakam, but with a player option for year 5.
Edit: I see you actually sourced your information from a somewhat reliable website. I guess I could be totally off here, but I really didn't think I was.