Mertz committed to Wisconsin 2.5 months before Kramer committed to Minnesota.
Cole Kramer (0.8266, DUAL 6'1" 180 lbs) made an unofficial visit with the Gophers on 2/16/17.
Then he camped with the Gophers which Graham Mertz (0.9129, PRO 6'3" 205 lbs) also attended on 7/29/17.
I'd venture to speculate that the Gophers gave Kramer a silent verbal possibly between the time of camp and his unofficial visit to MN on 9/17/17. Kramer may have wanted to look around a little. On 12/15/17, he declares the Gophers the front runner. He then commits on 12/21/17, and shuts down his recruiting. In the end, he truly wanted to be a Golden Gopher.
Mertz was offered on 10/4/17 and would have known about Kramer's silent offer. Mertz declared at that time that whichever school he committed to he want to be the sole QB in the class. So, he committed to WI on 10/8/17.
There is a lot of speculation that Mertz may still flip to another school. He hasn't shut down his recruiting. Guess what, on 4/26/18 the Badgers got a commitment from Marcus Graham (0.8111), a 6'0" 175-lb Dual Threat QB from N.C. Even if the Gophers got a commitment from him, the likelihood of him bolting for greener pastures is high.
Cole Kramer Timeline:
2/19/17 - UV to MN
6/17/17 - Attends MSU School Camp
6/25/17 - Attends Hawkeyes School Camp
7/29/17 - Attends Gophers School Camp
9/16/17 - UV to MN
10/21/17 - UV to NDSU
10/28/17 - UV to Iowa
12/15/17 - Names MN the leader
12/21/17 - Commits to MN
Kramer shuts down his recruiting
Graham Mertz Timeline:
2/19/17 - UV to MN (Same as Kramer)
7/29/17 - Attends Gophers School Camp (Same as Kramer)
9/16/17 - UV to MN (Same as Kramer)
9/23/17 - UV to OSU Cowboys
9/30/17 - UV to WI
10/4/17 - MN offers
10/8/17 - Commits to WI
12/17/17 - MI offers
1/12/18 - Coach visit, Harbaugh
1/17/17 - tOSU offers
1/21/18 - Ole Miss offers
1/24/18 - Mizzou offers
1/31/18 - GA offers
2/5/18 - ASU offers
2/6/18 - OSU Cowboys offers
2/19/18 - ALA offers
2/22/18 - Tenn offers
2/25/18 - UV to WI
2/26/18 - TAMU offers
3/1/18 - Oregon offers
4/18/18 - ND offers.