Fitting discrete curve to cosinus
As pointed out by Dan Boschen, your signal appears to have a frequency, which does not lie exactly on an FFT bin. Given that the signal contains only a single cosine, you can get a better estimate of its frequency by zero-padding the signal. Have a look at the code below, which reproduces your problem:

Fs = 450
f = 75.5

T = 1

t = np.arange(0, T, 1/Fs)

x = np.cos(2*np.pi*f*t)

plt.figure(figsize=(10,10))

plt.subplot(221)
plt.plot(t, x)
plt.xlim((0, 0.1))

f1 = np.linspace(0, Fs, len(x), endpoint=False)
plt.subplot(222)
X1 = abs(np.fft.fft(x))
plt.plot(f1, X1, '-x')
plt.xlim((60, 90))
plt.axvline(f)

f2 = np.linspace(0, Fs, 32*len(x), endpoint=False)
plt.subplot(223)
X2 = abs(np.fft.fft(x, len(f2)))
plt.plot(f2, X2, '-x')
plt.xlim((60,90))
plt.axvline(f)

plt.subplot(224)
plt.plot(f2, X2, '-x')
plt.xlim((72,79))
plt.axvline(f)

iMax1 = np.argmax(X1)
iMax2 = np.argmax(X2)
print ("Estimated frequency of the Cosine without Zero-padding: f=%f" % (Fs-f1[iMax1]))
print ("Estimated frequency of the Cosine with Zero-padding: f=%f" % f2[iMax2])




The top-left figure shows a part of the sine wave. The top right shows its fft (zoomed into the interesting frequency). As you can see, the FFT shows a significant values at two different frequency bins. This indicates that the actual frequency is between both bins. To find the correct frequency, we can apply zero-padding. Bottom left shows the zero-padded FFT, showing more samples in the frequency domain. The bottom-right shows a zoomed-in version and we see, that the peak of the FFT is exactly at the correct frequency f=75.5Hz. For more information about zero-padding, you can refer to an article I wrote about when you can apply zero-padding, when it wont help etc. 

The textual output of the code above is

Estimated frequency of the Cosine without Zero-padding: f=76.000000
Estimated frequency of the Cosine with Zero-padding: f=75.500000


As we see, if we take the maximum of the frequency bin of the non-zeropadded FFT the frequency is estimated to be at 76Hz. However, if we consider the zeropadded FFT, we get the correct frequency estimate.