Mixer LO to IF Isolation vs. 1*LO - 0RF Spur
The first table is only usable at an RF power of -10dBm, and only at an LO frequency of 5.25 GHz.
You would expect the power of a 0RF spur to stay constant, regardless of the input RF power, so it's not correct to try to use the first table to take 3.4dB with respect to a different RF input power.
Given that the LO to IF isolation is given as typical 32dB, minimum 15dB, quite a range, I'd say 1.4dB between the two tables is as near as you can expect to spot-on. Note that the 3.4dB is quoted at 5.25 GHz LO, the 32dB isolation is typical over the range 4.5 GHz to 6 GHz.

I see. Even though the values given in the table aren't valid for different RF powers, is it acceptable to use them as general guidelines while frequency planning? For example, some of the spurs are as weak as 90 dBc for the given test conditions. Is it ok to ignore those spurs even though the spur table will change with varying RF power.

The table 1 guidelines are to allow you to quickly eliminate mixers that are 'obviously' wrong for your application, they are not a substitute for proper receiver design. Once you have a shortlist of those that may do, you must then get either more comprehensive figures from the manufacturer, a physical model and test it at your power levels, or ideally do both.
The purpose of table 1 is to give you guidelines as to the levels of spurs to use when doing frequency planning. However, as it's given at one power level only, you will have to use your skill and judgement to estimate what it will be at other power levels.
In general, the nRF product will change at ndB per dB of signal level change, for typical diode mixers, when the signal level is small. When I've used RF mixers, they often have a table 1 taken at two different power levels, which allows you to determine whether signals are still 'small' at that level. For a badly behaved mixer, the spurs may become better or worse as you get to the 'big signal' regime, so extrapolation down to your low signal level is very uncertain.
Whether -90dBc is 'OK to ignore' is down to your application. If you're building an amateur data receiver, almost certainly. If you're building a commercial synthesiser, probably not. If you're building a commercial data receiver, then it's not going to affect your in-band demodulator performance, but what are your blocking specifications? You mention signal levels of -50dBm, which in my experience is low, even for a spectrum analyser, so you probably are pushing the spurious performance.