Admittedly, I have no accessible memory the presidency of HST, but after completing the late FDR's elected term ('45-'49) as he was FDR's elected VP at the time of FDR's death, the now incumbent HST was elected in '48 to a complete term ('49-'53), but was not a candidate for a 2nd allowed term. However, after becoming POTUS in Apr '45, and then continuing as elected POTUS from Jan '49 until that term ended in Jan '53, HST served about 7 yrs and 9 mos.
DDE, former US Army General, was elected '52 , and served 2 complete terms in office as POTUS ('53-'61), after re-election in '56 for his 2nd term beginning in '57.
JFK , former US Senator from Massachusetts was elected in Nov '60 and began his 1st term in Jan '61, but served only 2 yrs and 10 mos of said term ('61-'65) before being assassinated in Nov '63.
LBJ, as JFK's elected VP, was elevated to POTUS in Nov '63 due to the death of JFK, to serve the remaining 1 yr and 2 mos of JFK's elected 1st term. In Nov '64, the now incumbent LBJ was elected to his 1st full term as POTUS ('65-'69). During '68, LBJ announced that he would not seek, nor accept a nomination of his political party as a candidate in that year's election, for what would be his 2nd elected term.
RMN, a then former, but not incumbent VP ('53-'61), was elected in '68 to his 1st term ('69-'73) as POTUS. The now incumbent RMN was re-elected in '72 to serve his 2nd term ('73-'77). However, after the resignation of VP STA in Oct '73, POTUS RMN resigned in Aug '74, possibly to avoid being impeached and removed from office.
GRF, as RMN's appointed VP, was elevated to POTUS in Aug '74 after the resignation of RMN. The now incumbent GRF then ran for POTUS in the '76 election for a 1st term beginning in Jan '77, but lost in the general election.
JEC, former governor of Georgia, was elected POTUS in Nov '76 to his 1st term ('77-'81). However, the now incumbent lost in his bid for re-election in Nov '80 for a 2nd full term beginning in Jan '81.
RWR, actor and former governor of California, was elected in '80 to his 1st term ('81-'85), and the now incumbent RWR was re-elected in '84 for his 2nd full term ('85-'89).
GHWB, the now incumbent VP, was elected in '88 for his 1st term ('89-'93). And, he was a candidate for re-election in '92 for his 2nd term beginning in '93, but the now incumbent POTUS lost his bid for said re-election.
WJC, former Governor of Arkansas, was elected in '92, and again in '96, to serve 2 complete terms ('93-'01). Although impeached by the USH in Dec '98, WJC refused to resign as POTUS, but was not removed from office by the USS, and thus served his 2 elected terms.
GWB, former Governor of Texas, was elected in 2000 and re-elected in 2004 to serve 2 full terms ('01-'09). There were questions about the vote count in 2000, and some questions arose in 2004 as well.
BHO, former US Senator from Illinois, was elected in '08 and re-elected in '12, and is at present serving in the final year of 2 full terms ('09-'17).
Due to FDR's lifetime being before my birth, I do not count his presidency among those in my lifetime.
So, of the last 12 POTUSs, only 5, counting BHO, served their complete allowable terms.
I do not know what, if anything, this all means, but I do sometimes wonder, as I wander.