In the first circuit, the way it is wired, you are changing the mark-space ratio of the oscillator. In the second, you are only changing the frequency of oscillation, not the mark-space ratio, so it's always roughly
50%, no matter how fast it oscillates.
The 555 is a relaxation oscillator, it works by charging and discharging C1. When charging, it goes through R2 and R1, when discharging it'll go through R1.
Where the diodes come into play, is to help increase the variation in charge and discharge time.
when charging, it'll go through R2, D2 the right half of R1 to the cap C1.
When discharging it'll go through the LEFT half of R1 and D1 and to ground through the discharge.
as the wiper moves left, it takes longer to charge and discharges faster, giving short pulses.
as the wiper moves right, it charges faster, and takes longer to discharge, giving longer pulses.
The second circuit can vary the mark space ratio, but not as much. it'll just vary the frequency more as both the charge and discharge times get shorter with decreased resistance.
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